"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.

Size:

One category larger than the base creature. This notably affects AC, Space/Reach, attributes.

HD:

Double the base creature's. This notably affects BAB(and thus grapple and attacks), saves, skills, feats, and CR.

Speed:

Varies based on the type of the base creature's best speed. (Keep any other speeds the base creature has.)

Fly:

Replace with Swim speed half of old Fly speed.

Climb or Burrow:

Keep, and add an equal Swim speed.

other:

Replace with Swim speed equal to old primary speed.

Attacks:

Size up damage(in accordance with its size change), to-hit improves with HD.

Special Qualities:

Add Breath Holding. If the base creature had Scent, replace it with Blindsight 120'.

Attributes:

+4 DEX, others as affected by the size change.

Environment:

Terrain becomes Aquatic.