I also went with the waterproof lights, they work as advertised. The only thing about leaving the harness plugged in is unless everything is sealed 100% the saltwater will toast your wires quicker if you have the juice on. When making your connections for the new wires cut back the wire until you get a nice copper color, get rid of that black stuff it will cause you problems sonner than later. It's unlikely that you have tinned wire on the trailer harness but if you do keep it, the wire will be silver in color and it does not corrode as fast.
For attaching the new lights electrical tape and solder just won't cut it. Go with the butt end connectors that have the glue in them, Crimp 'em, cook 'em. The outside is a heat shrink material The glue inside will seal the wire to the tube and fill any pin holes you make when crimping.