The good news is that visitors to the National Aquarium in Baltimore's splendid Inner Harbor complex are spending less time in line to get in. The better news is that the aquarium is now on summer hours -- 10 to 5 Monday through Thursday, 10 to 8 Friday through Sunday. And the best news is that expansion plans are under way with a design competition for a marine-mammal facility.

Attendance is even higher as the aquarium nears its second anniversary in July, but the flow of visitors has improved, thanks to the staff's learning how to direct the crowds, and bottlenecks being eliminated in the constant renovation of exhibits on the tour.

Awesome, educational and entertaining for all ages, the aquarium keeps adding to its flora and fauna -- more than 6,000 now -- and some of the more spectacular of the charter denizens, especially the sharks and stingrays, have flourished and become hams. The effect of the spectacular finale to the tour -- the open-ocean tank -- is constant motion, malevolent and mesmerizing.

The tour is up, then down, five levels, with all the climbing on escalators or moving ramps, eminently navigable for all but the most infirm, especially if you skip the top level, the tropical rain forest, which is necessarily steamy (signs warn those with respiratory weaknesses away). Going up, the exhibits are essentially educational -- ecology, especially of this area -- winding up with the children's cove where guides help kids explore a coastal habitat and handle crabs, urchins and whelk.

The way down is through the hole in the doughnut- shaped pile of tanks, past hundreds of species of multihued and multishaped creatures darting in and out of an authentic re-creation of a coral reef, winding up at the awesome open-ocean tank and the pool housing two California seals, Bogart and Bacall.

Since you're at the Inner Harbor, and with energy left over, you can explore the lightship Chesapeake and the submarine Torsk on the dock immediately east of the aquarium, and the War of 1812 frigate Constellation on the dock to the west, next to Harborplace. ON THE WATERFRONT THE NATIONAL AQUARIUM is open 10 to 5 Monday to Thursday, 10 to 8 Friday to Sunday. Admission $4.50 adults, $3.50 students & over- 60s, $2.50 children. Group prices available. From Pratt Street, turn north on Gay Street half a block to parking garage. Ticket validated at the aquarium gives discount. Average fee, $2.50. Call 301/576- 3800. LIGHTSHIP CHESAPEAKE & SUBMARINE TORSK are open 10-5 Thursday through Monday. Admission (to both) is $2 adults, $1 children 14 and younger. FRIGATE CONSTELLATION is open 10-6 (10-8 after June 14). Admission is $1.75 adults, 75 cents for children 7-15, free for children 6 and younger. PEDALBOATS are available every day, 10 to 10, at $3.75 a half-hour or 47 an hour for two pedallers. ELECTRIC BOATS are $9 a half-hour, $17 an hour, with $5 deposit.