That could be fun. Honestly, I just really don't want it to evolve into a Water/Dark type, but it looks sorta evil aready. We already have Crawdaunt. --- THAT'S MISS HOME ROWED TO YOU http://i.imgur.com/a95OQPq.gif
Oh yes--I do have an idea for a fire/water pokemon, but definitely not a pistol shrimp. I'm thinking basilisk. Combine the real-world basilisk (a lizard that can run short distances on water) with the mythical one (some bestiaries claim that its gaze and breath don't just kill you--they cause spontaneous combustion), and...
I'm thinking in terms of the mantis shrimp's general pugnaciousness--nearly Primeape levels. Its usual reaction to anything is to swat it with one of its claws. Those claws can strike with the velocity of a rifle bullet, although certainly not the force of one (not nearly enough mass). Still enough to crack the shells of prey molluscs and crabs, though.
(Fighting or no Fighting, Brick Break should be an obvious move for it to have.)
Well first off, it isn't a myth that mantis shrimps can break glass because they can. There are two types, clubbers and spearers. My first peacock mantis shrimp (RIP) burrowed to the bottom of the tank and kept "clubbing" the glass until it finally cracked and split. It didn't shatter but the water leaked out and before I got home was completely drained. My new one is in a acrylic tank now.
Anyways, I could see a mantis shrimp Pokemon as either fighting/water, dark/water, or even fire/water. Mantis shrimps strike with such a force it actually causes the water to boil. They are extremely speedy and aggressive. Mine attacks a laser pointer light like a rabid demon.
Ability wise, compound eyes, swift swim, sheer force would be what I'd expect.
Stat wise, I'd hope for something with high attack and high speed based on an actual mantis shrimp. Probably low special attack and low special defense. Decent defense and decent Health is wishful thinking. Please GF make my wise come true lol.