During summer months in each city on island is possible to find good offer of all kind of water sports including Banana Bus, Parasailing, Water Ski, Slider, Fly Fish, Speed Boat, Stand up Paddle.
More about watersports available on this links:
Semi Submarine excursion
Interesting excursion with semi submarine from port Krk. KK 300 is a boat designed to look like a real submarine, including the conning tower, external steering and rounded shape of the hull. On the world exists only about 55 similar vessels. Semi submarine has the highest ratio of window to the traveler, it's small and nimble that it can take you in the best position for sightseeing. It do not dive below the sea surface, it has an underwater cabin with large bulletproof glass windows, which is located 1.5m below the sea surface. More information here