Sentret - Mainly (if not entirely) a combat pokemon. Every war needs troops.
Crawdaunt - A mix of combat, plus pure water. Kinda got f'ed there, since you already have Horsea and Sentret, but, ya know, having a back up as verstile as to handle both their talents (water and combat) is never a bad deal...I suppose...
Luxio - More water access, but mainly, electricty. With enough training, you could hook up the ol X-box and never be bored during your time in the zombie wastes. Plus you can bring the thunder, that's always cool.
Seismitoad - More water. Also, Acid Rain, that'll slow down those darn zombies!
With so much water on the field, I would get to a secluded island quick, and laugh at anything that tried getting past those water defensive pokemon. No chance. You're then stuck in my boat, what to do for food?
Anyways, that's my two cents on it.