This giantess scared the hell out of him! His heart is pounding and he feels like he is goint to faint. But the truth is there’s no reason to do this.
The giantess just wanted to teach him how to swim. It is an useful ability that everybody should have. And he doesn’t need to be mad at her for trying to help him. Well, there is a chance of drowning, but it depends on how much panicked he is.
Credit to Giantess-7of9