Wayne, if there is a pipe coming from below the "radiator" cap, then the tank you describe is definately acting as a reservoir, to collect coolant from the overflow pipe to prevent pressure build up and thus venting from the radiator type cap (which will have a psi rating). It does seem very odd that it is set below the level of the engine cap, but that may be due to poor installation or lack of practical alternative. You may find if you top up the engine filler to the top, that after running for a while, you lose coolant from the filler cap as it has nowhere else to go, and therefore the expansion tank is just like a "buffer".
Frank, the only exception to the rule that Simon (plesbit) has descibed so well is if your boat is keel cooled. A lot of earlier boats did not have a raw water side as such, but simply pumped the anti-freeze coolant via a skin fitting through some copper pipes mounted on the outside of the hull beside the keel to shed heat directly to the river. This was popular some time ago, but not so much now, as the vulnerability of the pipes made them a bit of a liability. Just have a look at the coolant pipes on your engine and see if they pass to a skin fitting, which would indicate a keel cooled set-up. If you do have this system, it could be a leak outside the boat and you would never see a wet patch.