Water weld is not soluble and will cure under water. It takes longer than if its dry though. I had no problems with it, like skimmer going crazy. From what I have heard using "reef" epoxy will make your skimmer go crazy. The water weld is much much much cheaper than the "reef" epoxy, and you get the same result
I had no deaths from using it, nor have I read about any. I'm not saying it can't happen though. With this being a new setup there will be nothing to kill though
I was worry about the same 2 years ago and decide to use live took because little critters that live in it help you system to speed up and all this time any algae break up or aiptasia and in the other part you can glue them together with epoxy angry the aqua escape you like.