Bodhi grew up on Jedha which is essentially a desert planet. It stands to reason that he never learned how to swim. This is fine for a while because being a cargo pilot really doesn't put him a position to need such a skill. It's a different story, however, when he gets dropped in a lake/ocean/river/etc. and promptly sinks like a rock.

Cue the panicked "holy shit, he's not coming back up!" moment from the rest of the crew and their reactions afterwards.