Dismantle isn't the right word, exactly.
Dismantle, you see, would imply taking the grate apart with in an orderly fashion, perhaps with a screwdriver or something.
This grate is several hundred years old, very rusty, and the most useful thing you have to hand right now is a flashlight. (Your not-actually-all-that-trusty hunk of lead is somewhere at the bottom of the room right now, and bouyancy is currently your top priority, so you're not diving for it.)
But it seems fortune smiles upon you, in a sarcastic sort of way, because after a few solid thwacks, a brittle corner gives way, and suddenly the whole grate comes swinging down, showering you in bits of rust, grime, and probably the remains of an ancient broken lock.
It's a struggle to ascend, but you brace yourself against the walls, pull with all your failing arm-strength, and heave yourself into the deluge.
The current takes you in an instant.