BALTIMORE -- When it comes to downtown waterfront development, this city tends to get the big things right. Witness: the World Trade Center, a pentagonal exclamation point raised at water's edge by architect I.M. Pei in the mid-'70s. Harborplace, an early-'80s success story and one of the Rouse Co.'s premier festive marketplaces. The "national" aquarium, a waterside landmark the instant it opened nine years ago.

And, now, the Marine Mammal Pavilion, officially unveiled Wednesday. Though dedicated to a questionable end -- a dolphin show -- the new pavilion makes an ideal architectural companion piece to the aquarium, a splendid fraternal twin.

To Peter Chermayeff of Cambridge Seven goes credit for the original, striking conception. His big aquarium building, puncturing the air with its sharp steel-and-glass top, hunkering down to the ground with its powerful concrete substructure, was just the right touch for its foreground setting on Pier Three. To James R. Grieves and Nancy Nes of the Baltimore firm of Grieves Associates goes credit for taking up where Chermayeff left off: Similar in forms and materials to the aquarium, their new pavilion on Pier Four firmly engages the older structure when seen from afar, and from close-up.

Washingtonians driving up for a look-see will get a great first glimpse as they leave the interstate to head east on Conway Street: Hovering on the skyline are two transparent steel-frame right triangles, their vertical edges parallel, locked in urbane conversation. The coupled image is satisfying in its boldness; the architecture of both buildings adeptly combines abstract geometry with the muscular building tradition that distinguished the harbor in its prior, industrial phase.

Differences between the two buildings, however, are just as critical to the pleasing effect. The signature triangular towers, for instance, are identical in outline, and they're lined up like mirror images. But the reflection is not precise: The white-painted diagonal struts of the pavilion tower play in poetic counterpoint to the aquarium's blue-painted upright grid. Likewise, the somewhat gimmicky porthole windows of the new building, lined up all in a row, play in staid rhythm in contrast to the aquarium's jazzier beat.

Overall, the new building is more open and transparent, especially when experienced from the inside. Different functions help to explain the contrast -- the aquarium is fundamentally an enclosed container, the pavilion essentially a house for a 1,300-seat amphitheater. But Grieves and company sought every opportunity to bathe this interior in natural light -- the aquarium's tower is used for a sweltering tropical rain forest display; that of the pavilion is simply a huge, commodious skylight for the theater.

The architects also capitalized on the setting to create tremendous views of downtown Baltimore and the harbor. The buildings are linked across a channel by a bridge with floor-to-ceiling glass panes at its center. Grieves says his intention was to enable the visitor "to emerge from the underwater environment to light, to the above-water experience." He exaggerates not a whit. Moving from the aquarium's darkness onto this light span is quite a thrill.

Plus, the panoramic vista from the amphitheater is in itself a dazzling show. The light-filled room is all-around spectacular. As is, of course, the prime feature: a trio of bottlenose dolphins (eventually to be joined by a pair of beluga whales) put through amazing paces in the impressive 728,000-gallon main pool, lined with eight-foot-high acrylic sheets so viewers can better appreciate the animals' underwater power and grace.

Though I loved the dolphins -- who could resist? -- I must report sadly that I found the show objectionable: Despite its educational veneer, it's a hucksterish attraction. The show is an excellent lure for tourists, sure to be a success, but even so, I'd rather see these dolphins from a distance on the open sea. Exhibitions are skimpy, once one has witnessed this spectacle: There are a dramatic life-size model of a humpback whale, a few hands-on exhibits and several modestly accomplished pieces of sculpture. The list of attractions also includes a fast-food restaurant -- again, splendidly skylit -- and a marine-mammal curio shop. Of more lasting value are a couple of classrooms and, still to come, an Animal Care and Research Complex.

No major city, with the possible exception of Boston, has more inventively transformed its once-declining waterfront into a civic asset. There's more in store too, and the talents of some of those involved bode well. Stanton Eckstut, the architect who helped design the superb open-space network for Battery Park City in New York, has been engaged to do similar duty for the developing eastern segments of the Inner Harbor. Richard Rogers, the English high-tech star, is at present designing the Christopher Columbus Marine Center to occupy the inland portion of Pier Five.

Baltimore is capable of the occasional dud, of course. The restaurant Harrison's at Pier Five, just a few years old, is a dull-witted piece of anywhere architecture unfortunately plopped in a privileged position on the tip of the pier. The old power plant, with its awesome smokestack silhouette, remains a puzzle -- it flopped miserably as a Six Flags amusement park several years back. Basically, though, the city's record for imaginative planning, forceful follow-through and architectural excellence remains intact. If the contents of the Marine Mammal Pavilion are disappointing, not to say unworthy, the architecture inside and out is a testament to an enviable local tradition.