There are several manufacturers of underwater connectors. Here are the names of some in the US:
AGO Environmental
AK Industries
Birns, Inc.
Burton Electrical Engineering
Cooper Crouse Hinds
D. G. O'Brien
Impulse Enterprise
Kemlon Products
Pave Technology, Inc.
Sea Con Brantner & Associates, Inc.
Subconn, Inc.
Underwater Systems, Inc.
The one I use most frequently are Impulse, Seacon, Subconn and Burton.