Cooling System (Fluid and Impeller)

[Acadia]

Couple of questions.....

Raw water impeller -- Is there a rule of thumb for when to change it? So many hours, years? I assume it is best to replace before it breaks.

I've had my boat now for about 1.5 years now and when I bought it I noticed that the coolant was in fact water (What appeared to be water could be additives not sure). Well I forgot about my water coolant until just recently and now we are getting ready to go on a bigger trip that will require motoring more than usual.

I am tempted to leave the thing as is because it runs perfect but I know I should probably flush and replace.

Engine: 1985 4JHE
Climate: Central Florida

Is there such a thing as using water with additives as opposed to coolant for whatever reason?

Makes me wonder why the PO did that

Any tips you can pass on or a fluid recommendation would be great!!

Thanks

[Chief Engineer]

The impeller moves the raw water...there are no additives you can put in it.

The fresh water side uses 50/50 antifreeze. Do NOT use anything Like Nal-Cool or any other additives many of them will severely...and I am not bs-ing you....catastrophically damage the aluminum in the engine.