You will be fine doing that. Solder and heat shrink is the best bet on
a boat.

No, that's absolutely incorrect. Solder joints are stiff... too stiff for
the moving environment in a boat. Unless the wire was supported, rigidly,
guaranteeing no movement, the motion of the boat will cause cracking, often
where the wire enters the solder. Once things crack, corrosion follows.
Or, worse yet, sparks jump the gap.

Second choice would be a butt splice with built in heat shrink
and dielectric grease in side.

That's the first and ONLY choice. Heat shrink is, of course, a must.

Use decent quality marine-grade crimped connectors and wire.