Never throught of marine applications - better add me to the list of amazed people!
When I worked in the electronics industry we considered the rosin fluxes as relatively passive for 'easy' jobs. Most rosins are able to remain on the circuit after soldering i.e. they are 'no clean', whereas most water washable types of the day were really aggressive to the components and absolutely had to be removed. The water washable fluxes were much preferred for soldering awkward materials, but there was / is a lot more specialisation in the WW fluxes making them very good for certain materials or combinations and virtually impotent for others. We used a WW flux for die-substrate attachment on a line making integrated power modules for VSDs. The Direct Bonded Copper substrate was nickel plated, and I forget what the die metallisation was. We needed very low voiding beneath the die to prevent thermal fracture of the silicon so a high performance flux was required. Other processes at the plant were migrating over to WW technology as the CFC regulations gradually banned our flux-removing solvents of choice. Once the processes settled down the results were actually better than they had been in the good old days of Chlorothene, Trichloroethylene, Methylene Chloride et al.
I had a play last night making joints between a nickel plated housing for a thick film hybrid and a bit of copper wire. Heat was from a propane torch. I tried using both a rosin plumbing flux and a WW plumbing flux. The latter was Fry's Powerflux and it was far superior in both the profile of the finished joint and the ease with which it was established. Although I didn't measure it, I'm certain the highly active WW flux allowed the joint to be made at a lower temperature too. To crudely identify the fluxes, rosin-based flux is typically a light brown colour, while the WW fluxes are often pure white pastes. Pop the lid off some in the hardware store or plumber's merchants.
Multicore Solders - now part of the Loctite group - used to have an extremely knowledgable technical department. If you are really stuck you could contact them.
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One day my ship will come in.
But with my luck, I'll be at the airport!