Thank you for providing feedback!
There is currently only two ways to get away from a boat-sized fish the player has to either swim far enough away or dodge 3 of it's attack for it to get bored and give up. We're looking to add new ways of interacting with it after the jam.
If you're talking about the giant fish it's indeed impossible to swim away from it, but must of the time it is benefitial to let it attack and eat you - this doen't do damage to you (at least it's not intendent - there is currently a bug when you can hit a rock if you're close to it, when the giant fish attacks) and you can catch more expensive fish inside.
Again thank you for playing!