| Maezer |
Pathfinder Adventure Path, Lost Omens, Rulebook, Starfinder Adventure Path Subscriber
Having a swim speed will grant you the +8 racial bonus to swim. They don't however get swim as a free class skill. Most creatures types get swim as a class skill though: Aberration, Animal, Dragon, Fey, Magical beast, and monstrous humanoid.
An example of a creature without a swim as a class skill is the Draugr from the Bestiary 2.
The Eidolon as an outsider would have to select it as one of his 4 optional class skills to get it as a class skill. (Unless he was of the aquatic subtype (I think this base form might have been in Ultimate magic) in which case that subtype would give him swim as a class skill.