Swimmers have large headcrests that resemble the Queen's headcrest. The body structure below their waist strongly resembles the structure of a fish, which mirrors the mer-creature that originated from folklore. The tails also have lateral fins and several trailing tentacles.
These Xenomorphs were first encountered by a group Colonial Marines platoon that was sent to the oceanic agricultural colony, "Bracken's World". Having become stranded on one of the kelp beds following an APC breakdown, the group was ambushed by one of the Swimmers who pulled one of the Marines underwater. The remaining Marines retreated to the APC and were able to fight of the oncoming Swimmers with their heavy artillery as more of the Swimmers join in the attack. Luckily, the Marines were saved in time by a Dropship and the Swimmers proceeded to return into the ocean depths.