I almost forgot my favorite aquatic with critter abilities is Leopard Tree Frog breed ID 5. Moveset (Tongue lash, Healing Wave, Frog Kiss) for pure undead countering I guess you would go with swarm of flies, but in general frog kiss is really nice.
This pet is the fastest frog in the game 325 at lvl 25 as rare quality. In addition to dealing lots of damage to undeads and taking reduced damage from them, his frog kiss and healing wave combo is good against a variety of pets, particularly tank like pets or pets that like to have long drawn out fights. Every time you frog kiss you have a 25% chance to make the pet lose 2 actions (if you attack first) and it also increases in damage with every attack. Healing Wave is just there to give you longer battles so that your frog kiss can get up to some epic damage levels. Not to mention the periodic polymorphs will definitely drag the battle out and give you 2 free turns basically. Try this guy out against a Mr. Pinchy 1 on 1 and you will start to see his role in your pet battles.
Note: The enemy team swapping pets resets the damage from Frog Kiss (not sure if this is intended or a bug, but does not seem consistent with the description.
Obviously avoid flyers like the plague.
Unfortunately frogs are very slow pets and even when you take flyers out of the picture there are still 110 pet/stat combinations that are faster than this little guy (at rare quality lvl 25). (that may sound bad but there are 836 combinations that are slower than him and if your opponent is the type that favors power then it is less likely that they will have a faster pet)