I like the idea of a point that's located in or surrounded by water.
The problems like others have said, demoman is OP and pyro is nearly useless with the current design.
One way to reduce demoman advantage on or near the point is to widen the distance between the cap point and the surrounding housing. He can place stickies on the walls of a small housing, and they will be close enough to kill anyone surfacing or on the point. If the housing is larger, and the surface area of water by the point larger, he must use stickies on the point itself or try to hit players with pipe bombs instead. Stickies on the walls will be too far away to do much damage to swimmers.
Another way to change the point area to help pyros is adjust the water depth close to the cap point. By making the cap point abruptly drop off into deep water, burning players can simply dive back in and come up behind the pyro on the other side. Instead of this, make the water around the point quite shallow for some distance around the CP, so burning players will take more damage as they retreat to deeper water. One way you could do this is put the CP on the summit of a sloping hill, and again, enlarge the CP housing to accommodate the deeper water leading up to it. The distance from the CP to immersive water should be measured so that MOST classes will take almost full afterburn damage before they reach the deeper water.
Both changes will also improve the space around the point so players can move more freely. You can still make the point primarily or solely reachable by swimming. Just make it so that when they get there, its not an ambush inside a closet.
I would add a second, dry land passage, one that is slightly tortuous and easily defended but off to the side a bit. This is so that attackers and defenders both have to go out of their way to use or defend this passage. A benefit might be that the passage continues on to the central point as well or has a secondary spawn in it. Or additional health packs.
As with water passages, beware of the temptation to be too generous with health/ammo, especially ammo because then the water areas and side tunnels become sentry farms.