He can't catch a glimpse of Leo, but Donnie's sure he'd just heard him. Figuring out exactly where is going to be difficult with all the water. He can't exactly access his computer right now, and with how disorienting things already are, accessing his computer the other way doesn't seem wise when he needs all his focus in the moment.

"Leo?!" he does manage to call out the next time he finds his head above water. It's slightly easier to keep so, which means less being thrown around by underwater surges. But the current's going faster, which has Donnie look in the direction everything's going.

...he knows that sound.

"Drop!" he warns, wherever Leo may be, hoping his brother can at least hear him before the water shoots them out and into the darkness below.