Re: how many cooling fluid are there?
There are basically two types of cooling systems on your typical pleasure craft:
1) Closed Cooling, or "fresh water" cooled is when the Impeller which is in the "raw" water pump brings lake water to a heat exchanger that serves as a radiator. The engine's "circulating" pump circulates "coolant" (antifreeze) through the engine and past the heat exchanger where the hot "coolant" exchanges heat into the cooler lake water by running through tubes that are surrounded by the cold lake "raw" water. Confusing, but there are two systems, cold "raw" water that cools the hot engine "coolant" . . . wheeeeew.
2) Open cooling or "raw" water cooled uses the raw water pump to pump lake water into the engine and the circulating pump circulates the lake water within the engine. This type has no heat exchanger as the thermostat simply opens when it needs cooler water (supplied by the raw water pump "impeller") and closes when it doesn't. This is less desirable especially in salt water as the internal engine cooling passages have that yucky water running through them.
