Use stranded wire
Don't use the standard Romex solid conductor house wire - make sure you use marine tinned copper stranded wire (such as Ancor). I would use 12 or 14 AWG for general wiring.Can you simply extend an existing cable run, or do you have to install a new circuit with breaker, etc? You might want to think about installing a GFI protected outlet. I would go with a plastic box if you can mount it - simply for possible corrosion avoidance.Derek