Mike,
Fear not- the raw water enters the cooler at one end, passes through the tubes then exits at the other end to move on to cool the transmission oil via its own heat exchanger then ultimately into the exhaust to cool that too. If you will excuse the pum, the poor sea water must be exhausted by the time it is spat unceremoniousyl out the exhaust
. The exchanger is divided into three separate compartments; the ends, where the raw water enters (in one end), and exits out the other. and the larger centre section where the coolant circulates around the outside of the tubes.
The two end chambers are connected by the tubes through which the raw water passes . The coolant circulates through the centre section and gives up some of its heat to the cooler raw water that is passing through the tubes. The raw water and coolant are totally isolated so taking off the caps at each end will just allow a little raw water to escape. Not too much. Don`t forget to close the sea cock although the heat exchanger is probably above the water line.
cheers