Zerxus has spent quite a lot of this vacation in the water. He didn't expect that, going in, but it's an easy way to anchor himself to the present: the physicality of swimming against the current, the flooding of his senses with an alien sea. That there are people to swim with - that's not something he'll ever take for granted.

It may well be perfectly normal for Jedao not to surface when a human should, considering. But there's no reason not to kick off the ship and dive back under, just to make sure; he can see so easily in the water now, and it takes a while for his lungs to start burning.