QUOTE
The Marine is just walking on top of Floor Gun Traps. If you can see closely between the two sprites on the temple terrain you can see that 1 tile is different from all the others and that a Floor Gun Trap is slightly on top of it.
Actually he doesn't use floor gun traps for the underwater part, all he used was Rock Sprites, Pylon Aura, High dirt for resistance, and Creep for the ground.

looks awsome.