Found with copper spitters from Pelagos, but I think it's better to report here; the spitters can walk on any tile, including lava, fluids from other mods, and the 'out-of-map' voids at the edges of the world, which doesn't seem intended for 'swimming' spitters.

oh god XD
I mean it makes sense based on how I made them
I just did not added colision to water lava etc

but I wonder how you discovered it?
they technicly should never spawn in conditions that this would be case. Unless I am missing something.

If you are working on mod that spawn them somewhere else or it is case that this could happen then adding extra colision should technicly fix it

Found the 'out-of-map' one because of Void Shovel, a mod that lets you 'dig up' landfill into 'out-of-map' tiles, then tested other fluid tiles with Editor Extensions' map editor toggle. (I usually use Waterfill instead of walls+turrets for defense, but obv that doesn't work for swimming spitters, hence Void Shovel)

there are also mod like Everything on Nauvis, which has lava on Nauvis, and ammonia sea potentially, would that mean they can swim on those tiles?

I think so but I have not tried it

tested: yes they do do spawn in lava.

thats funny