I think it was last night when I just saw a Reginald swim into my base like it was normal water. It doesn't happen often, but it's very comedic when it does.

lol peeper god

I have a glitch where if you build an alien containment under water, but build it upwards until it's above water, you can breathe air but still swim. The water level is in the tank, and can't be seen, but going through it triggers the usual "Splash" effect and underwater sounds, and going out gives the usual land sounds.

Relating to this question, you can also easily get fish to come into your base by holding a floater in the water, running into a wall of your base and letting go, pushing it into the wall, it will now float around the roof of your base like a sentient balloon.

I'm not so sure about flooding your base and then releasing fish into the water, before fixing the leak, but I think that might be a good way to get fish swimming through your base. Idk

Did you know that if you shoot a scanner drone into a moonpool you can drive it around your base (but only on that one floor)

I had a f*cking sea degon in my lava zone base

Sometimes a spadefish gets in my cyclops