In a game I'm in we have ventured underwater into the ocean. I play a summoner, my eidolon has gills, a swim speed, a Bite, 2 claws, and a tentacle attack. How would being underwater effect his damage output?
I've read through all the rules on underwater combat I could find however it didn't fully answer my question regarding natural attacks.
Underwater Combat
Land-based creatures can have considerable difficulty when fighting in water. Water affects a creature's attack rolls, damage, and movement. In some cases a creature's opponents might get a bonus on attacks. The effects are summarized on Table: Combat Adjustments Underwater. They apply whenever a character is swimming, walking in chest-deep water, or walking along the bottom of a body of water.
The chart after lists the effect of freedom of movement spell, swim speed and piercing vs. non piercing weapons, but I don't see anything explaining how does natural attacks are affected by being underwater.
Best Answer
The GameMastery Guide on Water says, "The rules presented in the Pathfinder RPG Core Rulebook for underwater combat apply to creatures not native to this dangerous environment [i.e. underwater], such as most PCs" (214). (Fortunately, monsters can be sorted by terrain.)
It seems reasonable that an eidolon that possesses gills be considered native to water, and, therefore, such an eidolon shouldn't suffer penalties when using its natural weapons during underwater combat, but you should check with your GM to be sure.
Also Paizo creative director James Jacobs in a 2012 post said, "Creatures that have the aquatic [sub]type don't have to worry about those [underwater combat] penalties[,] but all manufactured weapons must abide by these rules, regardless of whether or not the creature wielding them is aquatic." So, in conjunction with the GameMastery Guide's rule, if, for example, the eidolon opts for a greatsword instead its claws, the eidolon suffers the underwater combat penalties on attacks made with that weapon. (Note that Jacobs makes it clear that this is a ruling not a rule, but it seems okay and comes from an often authoritative source.)