Creating a waterproof project enclosure
As already mentioned, 100 ft is a lot.  IEC 60529, which is a standard for ingress protection, tests their IP67 rating at 1 meter depth for 1 hour.  IP67 is generally considered to be equivalent to a NEMA 6 enclosure, I think. 

There are a number of companies that make multi-hole, water-tight cord grips.  Two examples that come to mind are Heyco (IP68) and Sealcon (IP66/IP68). There are other companies that make similar things, too, so look around.

One thing to keep in mind is that if your lead wire exiting the box is stranded, you have a leak path through the wire (between the insulation and the copper strands).  One solution to this is lead wire manufacturers can fill that space between the insulation and copper to prevent leaking.  Another option is to have a water-tight connector on the other end of your lead wires, too.