The Swimming Alien is an aquatic Xenomorph.
All Xenomorphs can swim, but since these Xenos have no legs and only an eel-like tail to propel them, it can be said they swim better than their terrestrial cousins.
The Swimming Alien is much like a terrestrial one from the waist up, but from the waist down it has a larger, flat, eel-like tail instead of legs. The tail has small "legs" on it, but it is unknown if these legs are operational and capable of land locomotion.
The Alien itself appears to take on a blue color, hinting it may be of Warrior rank, though that could simply further drive the point that the creature is marine based. It may be a type of Praetorian, due to its jaggedly crested head. The Swimming Alien has shorter dorsal tubes than the typical alien (excluding the Runner which has no tubes at all) and thin pieces of flesh hanging from its face, although these may or may not be whiskers.