Ok, I'm doing a monowater so I can definitely help you. Seismitoad is actually available little after you get to terajuma (midgame, maybe even a bit earlier). As for politoed, he would be great, but I don't know of any king's rock as of now.
I would suggest a ludicolo, under the rain he's kinda the mvp, and tentacruel isn't bad either. I have to say though, Basculin is just amazing, and under rain he's kind of unstoppable. Swanna too actually surprised me, it can delete many threats. I would change seismitoad with Swampert (you find a mudkip a bit after him). Shiinotic also has rain dish, and can learn spore, so you may evaluate him instead of ludicolo to add some trickier pokemon (with fairy power). Hope this helped, if you have any other questions just ask