1983 Mercury 115 outboard 6 cylinder. What is the spark plug gap

50:1 But if your doing mostly low rpm cruising (trolling) then you could reduce that a bit to 75:1 to help reduce smoke. If you do mostly mid to higher rpm cruising, then 50:1 (1 pint to 6 gallons. )
Or 200mls in 20 liters if your Canadian.

The plug gap is most likely .040, however can you please confirm your serial number?

check compression in cylinders , ignition coil , if has distribuitor check also

Have your spark plugs changed AND carbs cleaned and adjusted.
Usually that can also be caused by a seized advance system ( plate under the flywheel, commanded to turn with the throttle linkage... )

fuel goes in at connector then to fuel pump then to carbs ! remove connector off fuel line and prime fuel up so you know you got fuel ! then reconnect to engine and prime up again till you see filter filling and bulb should harden , then remove a drain screw from carb bowl and prime again and see if fuel comes out of all carb drains ! if yes then start engine with or in water with kill lanyard on if ok then good ! not then check sparks on all plugs putting them against the head for earth and crank to check ! if ok then i would remove carbs and clean the jets etc ! compressions 100psi or more on all cylinders ! less means recon pistons etc !

Quiksilver marine dealers !

Clean or change the Spark Cables>
to cleab the pull them out and with antirust spray and clean

need year and make? Tools(1) A compression gauge (2) a spark gap tester.(3) A Clymer service manual for your engine. (you can purchase from your local dealer). I know all these are expensive but if your going to be a outboard owner you will always need them. Troubleshooting (1) check compression. without the year and make I'm giving you a average numbers. Look at your Clymer manual for specific specs. all cylinders need to be over 100 lbs and equal each other within 5 lbs.(2) check spark. use a spark gap tester all cylinders should jump a half inch gap. if you have good compression and good spark, fuel is the problem nine times out of ten .Ethanol gas is proven to stay stable in fuel systems for only 30 days in 90 + degree heat. Drain gas tank,clean fuel lines replace primer bulb and replace all fuel filters. Yamaha has a gravity filter under engine cowl clean it. Finally replace spark plugs, I know spark plugs probably don't look bad but this is a outboard. Fresh spark plugs means less resistance for the ignition system. The ignition system is week compared to automotive. Test drive boat, if the problem is still there then there is further steps to take. Again I don't know the make so I guessing what fuel injected system. Pull out your clymer manual. what is great about the Clymer Manual for Novice techs is it gives detailed info on your outboard. Troubleshooting tips and teaches you how your outboard works. without the test information I would guess the electric fuel pump. but test everything first because a mercury efi fuel pump will set you back 600.00

Your motor is fine. It needs to put out more than 12 volts or the battery will not charge. Most alternators put out between 13 - 17 volts, so this is well within the range.