I rebuild my own, but then I am/was a machinist, so able to do necessary repairs to the typically worn sector shaft. If your box only needs a reseal kit, it's doable by someone with good mechanical skills and necessary tools as well as access to a bunch of so-so YouTube videos!!. If there sector shaft looks anything like this one below, buy a new PS box.
Other than that, it seems that the better rebuild and upgrades can be bought from RedHead Steering.
I'm not sure how water would get in unless the sector shaft seals are totally worn out. If that were the case, fluid would be pissing out too. The only other entry possibility is at the input shaft seal, but the water would need to be pretty high for that.