Because the navy is all about their diversity cocksucking, and whether or not you can swim doesn't really matter if you're most likely just going to be stuffed into an administrative role to prove a darky or someone with a vagina (preferably both) can be given important jobs too :^).
As for the dumbo nignogs in particular that join knowing full well they'll have their non-swimming monkey asses dunked underwater in basic training, the navy is probably the best branch outside of the air force in terms of gibs and general work place stress because someone yelling at you for being an incompetent fuckwit is a raycis no no. Also the air force seems much harder to join because of red tape alone, despite being as pozzed as the navy.
As for the muhreens that branch has been aimed at the dumbest retards they can find to serve as meat shields since at least Vietnam, and since niggers fit that bill perfectly you can kinda see why there's a lot of muhreens who can't swim.
As for the coast guard I was unaware they had that problem. I was under the impression they're an even bigger pain in the ass to join than the air force.
For non-nigger people who join and can't swim its most likely for gibs in the navy's case, and in the marine's case a lot of morons join because they want to prove they're hardcore or because of patriautism. Also you get gibs on top of hooting like an ape and pretending you're protecting murrica by shooting at inbreds in sand castles.