Yes, all below the water-line... there are 6 of them below the water line.
Seacocks detest being left in one position for long, so the occasional cycle really helps them enormously. You really do not want them to stick on you, particularly sticking open.
Mine are nearly all the cone-and-seat type (well, "frustrum and seat" is probably more correct), and they only need a right angle turn. They also have a grease entry point on them, and that's useful also. Tun them to the closed position and inject some grease. It helps, but it's not quite as good as stripping them and greasing liberally.