How about 4000 urethane (i think it is 3M). I used to use that (or 5000) on various Sailboat projects including a polyethylene water tank that fractured in some rough seas. Fixed an 80 gallon tank with 4000 and it never failed even when green water was rolling over the cabin top and I was snapped onto my safety lines trying to figure out how I can make really dumb decisions!
If I remember right, the guys at West Marine told me "4000 for most underwater projects, or 5000 if you really hate the next guy that has to try to get it apart". The cure time is truly as long as it says on the tube, but it wont eventually let go like the silicone sealers do.