Klonan Thanks. Yes, look's like a bug of HoverCraft mod. I'll report bug report to author this mod.

Hello,
I noticed that group of biters near water trying go through the water even it's imposible. Then they get to the water they stop there.
I'm using only Bob's enemies mod, that can change biter behavior. I disabled this mod and saw, that this is raised by this mod.

Example without Bob's ...