If you can get your hands on a digital multimeter, when the engine is running you should (going from memory here) get something like 14.3V at the battery. 12.2V when off. (I'm sure someone will be along soon to confirm / correct the decimal bit)
I'm guessing you have asked 'coz it won't start?
Couple of things to check - if you are not getting 14.3(ish)V when running, chase the wires back and see where you are loosing voltage - common things are loose or corroded (copper will have tuned black) connections and if you have one of those "red key" battery switches check the in and out sides of it - they are about the least waterproof electrical item ever designed!
Start at the flywheel - there will be two wires (back to memory - will probably be corrected by someone with a wiring diagram!) I think they are Yellow, and lead to a box with at least one other wire (red) connected to it. (There may be a grey sharing a terminal with one of the yellows that goes to the tacho)
The red wire will likely go to the starter solenoid on the battery side (the + wire that leaves the engine). Most do - if yours is a more modern machine it may go straight to the engine management box - but as you say '87 I'm guessing yours isn't that new
(if the boat is ashore & you can't run the engine, start at the battery & check the voltage stays at 12.3 or whatever all the way to the engine. )
Other thing if it's an '87 model you may be suffering from the opposite problem.
Mine cooked a couple of batteries as the regulator was only that. If you have a wraparound cowl there is a chance you have a similar rectifier to mine (a diamond shaped plastic box about 1" square) - I have since replaced mine with a regulating one off a Honda, as the battery charge was something ridiculous like 19V at about 1500 rpm - and rising with revs. Mine now carries a honda combined regulator / rectifier, and hopefully will mean my current battery will outlast the previous two combined.
Needless to say that extra voltage does absolutely nothing good to your battery.
If you find black copper in the connections, just replace the whole cable - once that sets in it's a gonner.