Doing as Hyp suggested, giving it a 30 ft. swim speed and no sting attack, converting it to Animal type with Int 1 and extra skills and feats as a result. This gives:
Giant Crayfish: CR 3; Large Animal; HD 5d8+10; 32 hp; Init +0; Spd 50 ft., swim 30 ft.; AC 16, touch 9, flat-footed 16; Base Atk +3; Grap +11; Atk +7 melee (1d8+4, claw); Full Atk +7/+7 melee (1d8+4, claw); Space/Reach 10 ft./5 ft.; SA constrict 1d8+4, improved grab; SQ low-light vision; Saves Fort +6, Ref +1, Will +1; Str 19, Dex 10, Con 14, Int 1, Wis 10, Cha 2.
Skills: Climb +8, Hide +4, Spot +8, Swim +12
Feats: Improved Natural Attack (claw), Weapon Focus (claw)