Xenomorphine, babies don´t 'swim'... swimming is a coordinaton between arms and legs to make yuo progress on or under water... That is why babies have to have their mothers holding them... They can stay under water because of the 'womb' but not for long... water is not amniotic fluid... So, the don't 'swim'... just are able to hold their breath for a while... Swimming is much more than that... And animals like ducks and crocodiles ar not a good example because the are NATURALLY bred for water... That is why the membranes on ducks' paws and the 'aerodynamics' and morphology of the crocodiles... Aliens were not originally conceived to be able to swim, and you know that by studying their morphology carefully... check the paws, the claws, the general morphology of the body... The Aliens swam with their legs, and not with their paws, if you recall... And in Aliens we only see the Alien come above, we don't see iot swimming in any way... Just for scare effect... About your last remark, ca you consubstantiate that...? It sounds pretty farfetched to me... We ar amphibious...? We can't breathe under water, we can't even swim under water without flippers and respiratory apparatus, so where did you read this...?