"Aquatic" is an inherited template that can be applied to any creature that doesn't already have a Swim speed, and whose environment terrain is not already Aquatic.
One category larger than the base creature. This notably affects AC, Space/Reach, attributes.
Double the base creature's. This notably affects BAB(and thus grapple and attacks), saves, skills, feats, and CR.
Varies based on the type of the base creature's best speed. (Keep any other speeds the base creature has.)
Replace with Swim speed half of old Fly speed.
Keep, and add an equal Swim speed.
Replace with Swim speed equal to old primary speed.
Size up damage(in accordance with its size change), to-hit improves with HD.
Add Breath Holding. If the base creature had Scent, replace it with Blindsight 120'.
+4 DEX, others as affected by the size change.
Terrain becomes Aquatic.