i have used this mod for quite a while and sometimes when i start a new game, the biters can suddenly walk on water.
this issue is quite annoying when it happens but can't really be reproduced intentionally. or at least not by me.
disabling the mod then loading and saving the save file without the mod and then re-enabling the mod usually fixes the issue.

edit: nope, disabling-enabling doesn't work for the same seed, i learned this the hard way...

It is very much intended yes that biters can also traverse the water.

...yeah i just checked the information page in this modportal page and it's indeed written there... but i have to ask, if this is part of the mod, why is it not in the ingame mod description?

could you make the biter swimming a toggleable option?
i use this mod mainly to not accidentally drown myself when using waterfill, not to make a complete turret-wall a necessity, invalidating all resource saving choke points...

getting jumpscared by biters through this mod is the main reason i abandoned my seablock mod tries.
while i have found another mod that enables swimming, that mod does something weir to the watertiles and i don't like how that looks at all.

I can't. The biters and player are on the same collision mask. In the past when I looked at this the only way was very resource intense and I'd rather not do that. Also this is not a cheat mod and allowing you to traverse the water while not allowing the biters would make this a cheat (You might as well use /editor to traverse terrain at that point).

The actual code that makes water traversable is here: https://github.com/NoxyNixie/Noxys_Swimming/blob/master/data.lua#L12 up to line 17. All the other code is fluff for the particle and ripple effects.

i didn't intent to use it as a cheat mod.
well, i did try to lose chasing biters a few times by using water, but that was only a small shortcut to get behind my defense lines faster.
it's unfortunate that i can't use this mod as i intended but i guess there is nothing i can do about that.
thanks for the replies and clarification.