#8 sounds a hair large, Frank. NEC states that for 15 amps, #14 is called for. (Actually capable of more depending on insulation). # 12 would allow for enough leeway. Capable of 35 amps (but restricted to 20 amps in NEC).

Also, go to West Marine ( or other boating store) & get waterproof butt splices for everything. Much more expensive, but much better.
If you want an outlet for them cheaper, I can PM you one. Need to buy quantity, but worth it in the long run.

Ed