Reviewing the ABYC document E-9 DIRECT CURRENT (DC) ELECTRICAL SYSTEMS ON BOATS and searching for the term solder, it occurs only a few times. Here are some applicable sections:
From E-9.16 INSTALLATION, Section k., Wiring Connections:
(8) Solder shall not be the sole means of mechanical connection in any circuit.
EXCEPTION: Battery lugs with a solder contact length of not less than 1.5 times the
diameter of the conductor.
That's it for solder. No where in the ABYC recommendation will you find any advice to not use solder on a boat for electrical connections. Take a moment to ponder this: if solder were not to be used in a marine environment, how would all our modern electronic devices be assembled? They're all full of electrical connections made with solder.