Forcing AI's to only swim? [4.9]

I have a bunch of AIs that swim around in my water, which start chasing a player once they spot him.

They navigate around the water using a navmesh on the surface of the water and ~1.5 buoyancy that makes sure they stick to the surface.

I’ve disabled walking in their movement components in the ‘movement capabilities’ list and only enabled swimming. However, they completely ignore the fact that they shouldn’t walk, and just follow the players (or certain other AI’s) ashore. How can I stop my fish AI from doing that?

I guess the only way I can think off is to create a second nav mesh, one that covers just the sea and make my fish ai use that one. While any other AI can use the first nav mesh which would cover the whole world.

Seems a bit messy and might be a bit expensive to run two seperate nav meshes, I think?