I already asked your loyalist "brothers" this question

So I figured why not ask those edgy marines and see what they come up with

Can you lot swim?

The gods will just grant us fins and gills as glorious mutations!

We can bend the laws of time and space.

I think swimming isn't that hard.

In Daemon Worlds definitely, in Worlds were the Chaos gods have no power...no.

*Jumps into lake*
*Emerges from lake*

Ya

I imagine it would be a difficult task, as we would sink like a rock.

We have Chaos approved floaties tho.

I dun't mind ya spiky gitz takin' off your dakka and 'rmor ta go fer a gud swim, just dun't expect ya loot ta be dere when ya get back!

*grabs noise marines gun*
Why dis gun make noise 'stead of shootin' dakka? Dun't make no sense.
*welds a power sword on it*
Now dis is a betta' gun dis is!

Nu they can't like the other humies, they sick but it does not stop them from enjoying the underwater life.

I don't know if we can swim, but we can certainly bathe in blood.

Depends who is writing the book at the time.

They're certainly extremely difficult to drown though, that's been proven more than a few times over.