Do Slaughterfish need Navmesh like other NPC/
Does it have to be deep water navmesh?
Thanks!

(I understand pond spawn don't need water, actually, but slaughterfish are an actor type object.)

if its an actor, i'm pretty sure it needs a navmesh

And I assume that would be the water marked Navmesh ("O")

i would think so. it's probably what they use to determine boundaries of where they are allowed to swim

Yup. You must navmesh everywhere, including underwater. Use "O" as you said to make the underwater navmesh blue. This not only will allow slaughterfish to function, but it will alter how other creatures act around there. For example, NPC's will walk directly ON the navmesh surface if it's not marked as water. Meaning NPC's will grind along the bottom of your lake. The animation is determined seperately. So they will be swimming (Not walking), but they will follow the ground. However if you mark the area as water, the NPC's will swim along the surface instead.

How does it work if the water is only about hip to shoulder deep? Will they still swim id I "O" the mesh?

BTW, I am about to find out , as I have a subtrranean cave with water and will test this out. And report back.

Like I said, I think animation is determined by the water system itself, not the navmesh. (Which only determines where they swim and how their AI acts) So if the water isn't deep enough for them to enter 'swim animation' they would keep running.

So to be clear the navmesh goes on the ground below the water or on the top of the water it's self ?

water navmesh should NOT be on the water surface. It should be on the river bottom / riverbed - i.e., the ground beneath the water plane.

Here's what I found.
Like AV says, where it is shallow enough (below shoulder depth) the actors walk / run on O mesh. slaughterfish will have trouble swimming and attacking.
Deeper than shoulder depth - actors swim and slaughterfish have an easier time swimming / attacking.