To get to the gym leader, you have to go and investigate a sunken temple in a lake, which requires both Surf and Dive. Happily for everyone, this gym is the third one along the circuit, and the first two gyms give out those TMs.

Once you've found him, you return to the surface, and proceed to the gym. Oh, did I mention that the gym is filled with puzzles that you have to navigate twice? (troll.jpg)