Tinned stranded copper is best (more corrosion resistant than straight copper.)
I always thought the best splices were soldered and adhesive-lined shrink sealed, but the ABYC (American Boat and Yacht Council) now says that soldering is a no-no, as it creates a definite flex point where the solder ends, making failures more likely. They suggest crimps (I suggest using adhesive lined shrink crimps), though, personally, I see little difference in the flex of the wire at the exit.
For general wiring tips, try to eliminate spots where water can wick its way up the wires; you can use silicone grease to seal things up if the wiring is somewhere that is out of the way - messy if it isn't out of the way. Grease also makes a good corrosion barrier on exposed terminals.
If you can't find adhesive lined shrink tubing, a hot glue stick can be shaved, and the shavings stuck in prior to shrinking regular heat shrink. Seems to work OK for me.
You should ideally run pos and neg wires, but from a couple of bus bars or a distribution panel (which you can get complete with switches and breakers or fuses); will make things a lot simpler to wire.
I would personally run wires back for your radar thingie. Have it fused, as well. If you wired it to the engine, it would not be powered while sitting on the hook with the engine off. On a switch from the battery, you have a choice.