If it is a Volvo Penta AQ230B then we know it is NOT a 3.0 GM block
Those engines have closed cooling, meaning engine coolant/antifreeze flows thru a 2 chamber heat exchanger and raw water from the lake flows thru the other chamber to keep the engine cool. All of that is a good thing because raw lake or sea water will corrode the aluminum heads quickly. Also with closed cooling the engine can run a little hotter and maintain good combustion efficiency. Slightly easier to winterize also. The engine is based on a Volvo automotive block but there are differences, I do not know the specifics there, sorry.
As with any older machinery, prior maintenance is paramount. There are specific maintenance procedures, all from the Volvo Penta operator's manual for the AQ230. If no shortcuts were taken then that will be a good setup. Those drives are very stout and maintenance items are simple enough for the mechanically inclined. Parts are sill available for that vintage of drive. Changing the impeller for the raw water pump, a 2 year maintenance item, is easy because the pump is on the front of the engine. On Mercs and others the outdrive has to be taken apart, just one thing that is simpler with a VP.
If you are not inclined that way and need to depend on a shop to take care of things then look and call around to see if any shops close to you are VP shops, but make sure they know the older "white drive" units as they are
way different than the current VP units. Along this train of thought, be wary of the "marina" that said it is a 3.0 GM. You did not say what role the peson played at the marina, if a sales person then OK, but the 3.0 block comment indicates a sad lack of knowledge for that boat's driveline.
