Following up with your head gasket or cracked head idea, it's possible your two problems are directly related, with the mixed sea water in the coolant causing the oil in water contamination. here is a scenario to potentially check out with your mechanic.
1) I assume the heat exchanger is old, perhaps original, and is now leaking raw water into your coolant (fresh water system). They all fail eventually if old enuf...and tend to do this.
2) if it is the heat exchanger, sea water will typically inject into the coolant. This will dilute your coolant, making it less able to withstand freezing winter temperatures during winter layup- possibly cracking your head, which leads to salt
water and coolant mixing with your engine oil through the cracks in the head and journals leading to the lubricated surfaces, during operation in the spring. Marblehead Maine
temps got down to -6 F last January, so certainly cold enough to freeze diluted coolant in the engine, if it wasn't drained, cracking the head.
3) alternatively, upon start up this spring, the diluted coolant overheated the engine, causing the head to warp and blow out part of the head gasket, same result as #2 above.
4). Salt water in the engine oil, regardless of cause, is a bad thing, as the salt starts to corrode and score (sp??) the journals, bearing surfaces and engine parts, etc. So, at a minimum, and regardless of the head/headgasket issue, flush the engine repeatedly with fresh oil changes- up to five or six times soon as you reach operating temperatures - to eliminate any salt residue from internal engine surfaces. This will help save the engine from damage caused by salt water ingestion.
An easy way to check for a cracked head or head gasket problem is to run the engine up to normal operating temps with the 'radiator' cap off the expansion tank. If you see air bubbles roiling the surface of the coolant, or coolant starts to smoke and boil, you know you are getting superheated coolant out of the defect in the head. Combustion in the cylinder blows heat of combustion out through the cracked head or head gasket into the coolant stream, causing air and boiling coolant to circulate into the expansion tank, where you see it bubbling off into the atmosphere.
If no boiling or bubbling in the coolant resolution, your sea water mixing problem is elsewhere in the engine - maybe the raw water pump
as others have suggested. I've never had a pump
that could do this, as all my engines have had only a shaft driving an external pump
Anyway, it would be very interesting if you could post the outcome! Good luck.