My first thought would be don't splice as this is a marine environment. I try to keep everything a continuous run. Here is a link http://www.knowyourparts.com/technical-articles/hand-wire-splicing-techn... showing a splice for 4 gage wire. I checked Don Casey's  Electrics Simplified and he shows a similar method for single strand, nothing on stranded. In reviewing though I noted an interesting point about butt connections that made some sense. In a butt connection you are only crimping the exterior of the wire, meaning you are not getting full contact through the complete diameter of the wire.................one would assume a drop in voltage.

Continuous run of the proper size wire is always best but if your going to splice two wires to make the run  use one of these instead of a bolt to connect..