cuzisthe1,
Wow, interesting repair. If a repair is being made under water, the only real option is mechanical fitting. Depending on the DR you may need a stiffner inside the HDPE pipe. This will prevent the pipe from toe-ing in and allow for restraint.
Again, make sure that the fitting is a "restrained fitting" and that it actually grabs onto the HDPE otherwise it may pull out. Some of the manufactures acheive this restraint by encorporating a full circle "grip ring" into their designs. Stay away from point loading the pipe (screw type restraints).
I would recommend checking with the mechanical mfgs listed above for more details, they should be able to tell you whether a stiffner is required or not. If it is a heavy DR like 9, they may not require the use of the stiffner.
Good luck.