Eh. dunno. It's kinda like a big version of the PPG from B5. Not that great for rain, etc.
I think a plasma cannon like they had would just not work that great in water at any range other than VERY SHORT. Seems like it'd cool it down or end up burning through a ton of energy to boil/vaporize the water around it.
I'm thinking way too much about a movie that had a plasma cannon with a clip :lol
The way it works in my head, is if the Kaiju invade from water, and the plasma cannons they had didn't exactly do great IN water, it wouldn't make much sense to have em up on the wall. Might be why they seemed to just beat the crap out of the thing first, and use the cannons as a coup de grace.
Also, in regards to putting em on the rift, the rift was basically a big chunk of the pacific ocean. It's why various continents were attacked, ranging from San Diego to Alaska to Australia. It wasn't a case of "Oh, the rift is right here", and they had already mentioned trying to nuke it back when the whole thing started. It didn't work well.
I'd love some EU novels or tech specs to give a bigger background to how the Jaegers worked and how the war went. The movie stepped over a LOT of history to bring us the first big failure of the program.
Chris