When a skeleton is attacked by another mob (including another skeleton), it attacks that mob, reorienting on the player once the mob is dead. Skeletons sink in water and cannot swim, but do not drown.

Can skeletons swim DND?

These creatures have a speed and “movement can include jumping, climbing, and swimming.” There is nothing in their stat blocks that says they can't nor is there anything specific to undead that says they can't.

Are skeletons immune to poison?

Skeletons are immune to the effects of Regeneration as well as Poison.