My dad bought a 1966 Marktwain (150 Mercruiser) in 1966....
I was kid then and we had a lot of fun with that boat..
I brought it up here to the NW in 1997 and ran it in Lake Washington and it seemed to over heat at low RPM but was more or less ok 500 or more rpm above idle.
I called my dad and asked him when the last time he changed the raw water pump...........He said raw water pump?

(When I changed it there wasn't much left of it...)
I am surprised that it pumped at any RPM!! He used to pound on the thermostat housing thinking it was stuck closed!!
The pump lasted so long because he NEVER ran it dry I suppose....
My brother (who formerly owned my 211 Lib) Bought a new Ski Centurion in 2000. (Mercury 300hp 5.7MPI with Bravo raw water pump) He called me last Jul from the middle of the lake on his cell phone to ask me how to change the raw water pump.... That one went 8 years.
Last summer I Helped a friend change his (94 Bayliner) Mercruiser 3.0L engine (after he allowed it to freeze the previous winter) He bought the boat used from a dealer. He had the boat for 10 years. I asked him when he changed the raw pump........

(Another deer-in-the-headlights look) "what's a raw water pump"
That one went 10 years or so...
My 1997 Mercury Marine Owners Manual indicates:
Quote:
"Sea Water Pickup Pump" - Disassemble and inspect: "Whenever insufficient seawater flow is suspected. (if operating temperature exceeds normal range)
By that I guess I could run it until the temp alarm goes off!
I guess what I'm saying is that you could let it go for quite a while but at some point "Murphy" will be riding with you and it will fail someplace bad.
My brother had to change his in the middle of the lake with a crescent wrench and a pair of pliers

water was spewing everywhere!!.....The raw water pump on a Bravo engine is like a Volvo.....on the engine and he had a spare (luckily).....
If you have a Mercruiser Alpha you're not so lucky. you have to drop the lower unit to replace it. Not hard to do (unless you're at the lake!!!)
OMC Cobras aren't hard to change in the water......just don't drop the bolts or other parts.....
Bottom line? have a look at it every few years or so. Since they're fairly cheep.....it's just prudent NOT to put the old one back in....carry a spare too maybe.....
If you run it dry for more than a few short seconds, change it.
(Sorry for the long post. My wife doesn't ask me what time it is anymore

.....she knows how to build a clock now!!!)
Cheers,
Rick