Changing out (old for new) Solenoid on my trim pump for engine hatch........The one already on the trim pump appears to be the same as a car solenoid like a 90's mustang.
Does it make any difference if the new one should be marine type or just replace what's been there for the last 25 years?.......

"MARINE" electronics are spark protected.
In an enclosed bilge with a gasoline engine, an open spark can ruin your day.
Sierra branded marine solenoids are in the $25 range, so there's no reason to dip into your kids' college fund to keep your electrical gear marine-approved.

M

I agree with things that do have open sparks like a distributor and alternator and starters , as those have spark arresting screens built into them. I have pulled my original trim and main solenoids off my boat and opened them up to see what happened and they are no different than a std Ford starter solenoid. The only difference you need to be aware of is some ford solenoids have a ground post on them and some are grounded by their mounting bolts.

I would not consider an automotive replacement. They are not terribly expensive from Mercury. If you are replacing one, replace them both.