What electrical connectors to use underwater?
I have come up with an idea for an inexpensive underwater connector system, using PEX swivel adapters, that might be of use to you:

http://edwardmallon.wordpress.com/2015/01/29/a-simple-diy-underwater-connector-system/

Those adapters are rated to 100 psi, so should be good to a reasonable depth. The key is scoring the inside surfaces of the barbs with a wire brush so that the epoxy that seals the barb onto your cable gets some good mechanical adhesion. Be careful not to hurt the oring seats though.  I also make sure that the wire insulation does not pass continuously through the epoxy, so that water can not wick down the conductors into the join if you accidentally cut jacket/insulation on the cable. 

Links to the parts I am using are in the post, but there are plenty of companies making the parts. I will be testing these guys at depths up to 30m in a couple of months, as part of some new sensor assemblies that I am building. They will be left underwater for several months, but that means I won't have the final word for you on how well they handle long term depth exposure till mid year.



Addendum 2015-0-901

I just thought I should add a note that the connectors made it through their first real world tests: spending more than four months at between 7-10 m depth:
http://edwardmallon.wordpress.com/2015/08/26/field-report-2015-08-12-success-with-ds18b20-temperature-strings/

We put both of those loggers back in the water, with one unit stretching from 10 to 20m. We will leave them in for 4-6 months on this run. Will post that update next year.