Welllllll.......
It can be a pain. In your case, it's OK, since it sounds like you trailer your boat everywhere. Yes... if you add your oil first... pump fuel... and then drive to the docks... there is a high probability that your oil will get mixed.
BUT... there is a chance that it could be sitting on the bottom of your tank!!! (especially in colder weather)
Personally.... I would do it a little different.
I would take a 2 gallon fuel can with me to the pumps. Then... add your fuel to the main boat tank without "Topping off" the tank. Then... I would add the right amount of oil to the small fuel can, and add a gallon or so of fuel to that. THEN.... shake it hard to mix the oil and fuel together, and pour that into the main tank. At least, this way... you know the oil is already mixed into some fuel. Then... when you pour it into the boat... it will simply disperse. (This is known ans "Master batching" in the chem world)
Now.... were it becomes a TOTAL pain, is if you leave your boat on the water. There really isn't any way to assure that the oil is being mixed into the main tank.
Without a doubt... the early Mercury oil system is questionable... especially when it gets to the 10 year mark. But, if you are doing a rebuild... just replace the drive gear, with the stronger, new style gear, and don't worry. (The rest of the system is good)