Try Mr. Sticky's Underwater Glue.
I suggest some sort of silicone sealant designed specifically for continuous underwater use. A pool supply or pool maintenance company might be able to suggest something.
Normally, I would say JB Kwik Weld will bond to pvc very well if you rough up with coarse sandpaper and clean it thoroughly before applying to the plastic. However, I really have no clue how long will work if continually exposed to water.