If you have pushed a gallon of water out of the expansion tank overflow, and levels still seem normal and the engine doesn't overheat, it's likely that you are getting salt water
intruding into the freshwater/antifreeze side of the cooling
On the 2040 engine the salt water
side of the system is isolated from the freshwater side of the system by the front most and rear most hose clamps on the rubber heat exchanger end caps. There are two hose clamps on each end of the heat exchanger. The inner most clamp on each end seals
the expansion tank and prevents antifreeze
from escaping from the engine. The outermost clamp seals the salt
water side from the freshwater side. If one or both of the outermost clamps are loose, the possibility exists that sea water could make its way from the sea water side to the freshwater side filling the system completely and allowing no room for expansion. Then when the engine began to warm up the building pressure would force water out the overflow.
As mentioned by another poster here, blockage in the heat exchanger tube bundle or exhaust elbow
can cause higher than normal pressures in the seawater side of the system, and could contribute to the crossover.
I suggest removing the heat exchanger end caps and cleaning
the heat exchanger tube bundle. While you are at it, check the exhaust elbow
for water and exhaust flow restriction. Then when you reinstall everything be sure the hose clamps are positioned correctly and tightened securely, but not overnight.