The lowered water levels help Leo get his own head up above water, and in that time he just manages to see the remaining light from Donnie’s battleshell before he’s sucked under again.

It’s enough to orient him a little when he resurfaces next, taking a big breath before calling out, “Donnie!” and going back down. He tries to maneuver against the crush of water to get in Donnie’s direction, but everything is moving so fast…