That expansion tank will work just fine.
Do NOT fill it full to the brim as when the coolant gets warm it will expand and some will vent to the bilge, no big deal.
The vehicle style expansion tanks , that keep the radiator full , is done to attempt to remover all air from the cooling system.
This makes it more efficient and most car / truck mfg do make the radiator 25% smaller , to save material costs.
Boats with heat exchangers are usually created for worst case , full throttle , 90F+ water.
It is far more important to keep the coolant fresh and to clean the system with 2 part cleaner as the mfg suggests , than worry about weather the pressure cap can refill excess coolant.