The 500EFI is standard with raw water cooling, what ever you run in gets circulated throught the engine and exits the exhaust.
the 496 is closed cooled meaning the raw water comes in and passes through a heat exchange (big black thing on front of engine) then it passes thru and out the exhaust. The engine block itself has coolant inside and isolated from the raw water. Similar to a automobile whereas the coolant is cooled by a radiator but saltwater is sill going throught the heat exchange and exhaust. The exhaust area seems to be the most effected bacasue of the high tempratures and salt mixture. These if they fail will take out your motor. They need to be flushed every time.
You can get closed cooling put on the 500's maybe 1K each. but you still need to flush to protect the drives exhaust and heat exchanger.