Opening a major aquarium can be as dicey as paddling with sharks, to hear Rob Mottice tell it, and he ought to know.Mottice, as curator of husbandry, has been working to get the National Aquarium in Baltimore ready to open this Saturday; the other night he nearly became dessert in the shark tank. "That's what happens," he says with a shrug, "when you're cramming six months of work into one."

The bearded parasitologist from Ohio recently put aside the care and feeding of some 5,000 specimens of fish, fowl and neither, to show a visitor around. Next day he would have his little adventure, but now he played tour guide, at least when not answering his beeper, conferring with frantic staffers over the arrival times of animals and, at one point, ordering the removal of a just-expired eel.

A handful of American cities boast full-blown aquariums, but Baltimore, come Saturday, might reasonably boast most. Not that Washington's aquarium threatens to follow the baseball Senators -- though that won't be decided till fall -- there's another reason to head for Baltimore's Inner Harbor. (Though the disappointed director of the Washington aquarium, Craig Phillips, won't be at the opening. "I don't like to cry in public," he says, "and I'm not sure that I wouldn't.")

The first new aquarium in more than a decade -- Boston's New England, designed by Cambridge Seven Associates, the same architects, opened in 1969 -- Baltimore's is also the nation's only one with a rooftop tropical rain forest, stocked with the likes of macaws and parrots and sporting a waterfall that would do Disney proud, plus an unmatched view of the harbor.

What's more, its million-gallon habitat holds a huge dolphin "tray" and two doughnut-shaped tanks, a coral reef and shark ring one atop the other, into which you descend down a ramp toward the mysteries of the deep. And for good measure, it vies with Chicago's 51-year-old John G. Shedd Aquarium as the largest in the country. ("We'll have to get that straight one day," muses Shedd director William Braker.)

From its berth at Pier 3, cattycorner from Harborplace, the National (a designation Congress gave in 1979) looms over the water like a cathedral -- actually an apt word for the vaulted, glass-topped building. "With cathedrals, the internal organization has always been the first priority, and content is the prime determinant of form," says Peter Chermayeff of Cambridge Seven in his architect's argot. "So we conceived this thing from the inside out, whereas most buildings are looked at as an enveloping shell for a rather unspecific content."

In a flight of corporate fancy, Cambridge Seven also graced the place, inside and out, with blue neon fringe -- a squiggly wave that shapes a striking sight. "You might say it's our signature," Chermayeff allows.

The first exhibit you'll come across, next to the Harbor Seal Cafe, is the outdoor seal pool, where the mayor of Baltimore vowed to swim if the $21-million aquarium opened late. As things turned out, it was a month behind schedule, so William D. Schaefer manfully took the plunge, pledging to return this Saturday and tread water for the duration if the aquarium's still not ready. He'll be ribbon-cutting instead.

Once you climb the flight of stairs and step inside at Level Two, a visit can last three hours. Everything's deep blues and grays, which, along with the recorded sounds of surf, fish-talk and such, give the place an appropriately watery feel. At once you see the dolphins -- though there'll be no "Sea World" shows -- and catch a sidewise glimpse of the sharks; a promise of things to come. The suggested route, started at a colonnade of bubble tubes, takes you: to the Habitat Theater for a film on aquatic life, past color transparencies of such wonders as "Eye of a Squid," and, on an escalator, past an 1880-vintage finback whale skeleton to Level Three, into a large exhibit called "Maryland, Mountains to the Sea." It's a well-labeled display of fresh- to salt-water life from an Allegheny pond to the continental shelf, replete with anchovies in their natural state and an engaging little creature called the featherduster worm. In a few cases, you can even get your hands wet. Two levels up, you can actually walk through the Children's Cove, a display of a typical New England coastline area with crabs, shells and starfish to touch.

From Level Five, where lurk a gila monster and other tropical reptiles, you ride to the steamy rain forest, whose Amazon feel, what with orchids and some 40 roosting birds, poses an unsettling contrast to the harbor beyond the glass. Then, after soaking up the wrap-around mural of an ocean horizon, it's down into the Atlantic Coral Reef, 335,000 gallons stocked with 3,000 specimens of fish; then a leisurely stroll into the shark ring, and finally an ecological exhibit called "Man and the Sea."

Even with crowds, the spell would be hard to resist.

But back to Rob Mottice and the incident in the shark tank. As the man responsible for stocking the aquarium, dispatching on one day an expedition to Iceland for puffin birds and on another a mission to the Florida Keys for giant grouper, he's been laboring 80-hour weeks, only sporadically catching sleep. So it was in a state of dream-like frenzy, sustained by sheer force of will, that Mottice confronted the arrival of an eight-foot sandtiger.

The 250-pound shark, a savage-looking but sluggish beast best suited to the ocean's bottom, was doing poorly after a 15-hour truck ride from Montreal. An hour outside Baltimore, despite an infusion of oxygen, the animal had begun to belly-up in its coffin-like box. "That's not a good sign," Mottice notes wryly.

When the truck pulled in it was late afternoon, and Mottice and crew were ready at the service entrance with a forklift and a change of synthetic seawater, the latter to remove the harmful urea compound that sharks in shock secrete. Working quickly now, in controlled panic, they wheeled the box into a freight elevator, rode up to the "shark introduction area" on Level Two, slipped a stretcher under the animal, and hoisted it mechanically into the water. Mottice and aquarium director Jim Kepley, wearing wetsuits, were there to receive it.

Mottice grabbed hold of the animal, one hand under the stomach and the other behind the dorsal fin on a place called the caudal peduncle, and gave it a shove into the 200,000-gallon tank, where five adult lemon sharks bided their time. The new shark started swimming, but whirled frantically about, smashing into the glass, then stalling out and sinking. Mottice ventured in and gave another push, but the same thing happened. After several more attempts, each less promising than the last, the only thing left was to swim the beast around.

"I went in with my snorkle and flippers and mask," Mottice says. "I grabbed him and kicked my legs, moving his tail from side to side with one hand. We went on like this for a while. Then we rounded a corner. There were the five lemons, swimming all together right below us. As soon as they saw us -- me and this huge shark -- they went into what I'd call an extremely aggressive posture. They'd lowered their pectoral fins to an almost vertical position, which is what they do when they're about to attack. I had one thought: 'Oh, no.'"

It was at this point, he says, that he hugged the sandtiger tighter and gasped, "Come on, big fella, let's get the hell out of here," and kicked his flippers to beat the band. Then he blanked out. "I just couldn't think about the lemons. All my thoughts and energies had to be concentrated on reviving this sandtiger. So I did that and forgot everything else."

He swam with the shark for an hour, and the lemons miraculously kept their distance -- more frightened than mad, it developed. But the salvage operation wasn't working: Whenever he loosed his grip, the animal would sink. So Mottice returned the shark to the introduction area, where staffers blasted water over its gills with a hose. The shark puffed up with air. Mottice burped the creature -- "a big, deep burp," he says, but still it didn't help.

As a last resort, Mottice and company tried shocking the beast back to health with a car battery, hooking one wire to its chest and the other to an overheadrail. "We revivied him a couple of times, but in the end. . ."

The sandtiger died at half-past midnight. There was nothing else to do, so Motice went out for a beer.