One of the learning curves I had to get up to speed on was making waterproof electrical connections at depth. There are two ways that I found helpful to doing this.
Bulkheads and penetrators.

Bulkheads are (usually) female connectors with some type of sealing gasket that allow a male plug to attach and mate. 

Penetrators are generally wires which get fed through a physical passthrough and potted with epoxy to make them waterproof.

I am trying both on this project.

First I tried to make some penetrators, I resin printed some penetrators from a CAD file i found online. 

Cleaned up the threads with a tap and die set and then potted my cable.

Most professionals use Loctite DP420 for potting, but I choose to use a suitable knock off of Infinity Bond EP420 which is much cheaper at 16$ for 50ml and has similar properties.

I kinda doubt that this is going to work at really deep depths but hopefully they hold up for a hundred feet or so underwater.

As a secondary that I know will work, I like using bulkheads from Blue Trail Engineering. They are relatively inexpensive for bulkheads and are really nice.