hi guys, worked at schooner creek boatworks for a couple of years as the systems engineer/electrician on repairs and new installations/upgrades...
saildog has given the correct procedure- especially the marine grade heat shrink- Ancor makes it- it has a glue on the inside that will bubble out and really seals the connection, plus helps prevent flex. I do it for all but-splices and terminal ends.. ABYC says at least 1" onto wire from lugs which terminate battery cables too.
When I first started working, I asked about soldering because I came from a consumer electronics background- I was told that two things are at work: (1) is the safety of the mechanical bond...obviously the crimp just has more material involved in the connection and allows for more movement than solder- the solder with antimony etc forms more of a crystal than a metal and is therefore not a flexible material... but as I'm sure we've all seen on boats the wire strands broken at the crimped end- usually on a spade terminal but sometimes on the wires butt-spliced running wild on the boat. We could not have greater than about an 8" free run an anything and that was max max- a wire or hose and wire bundle might be approved to span something but always with support from a nearby surface... flex, solder or crimp is bad! (2) is dissimilar metals leads to galvanic corrosion... the joint and surrounding metals will be leached of electrons/molecules until the molecular lattice just fall apart... anyway, i think i worked on boats too long, lol, cheers!