And it is possible to do all this while the boat is in the water?
I'm assuming you have to do it from a dinghy.
Sounds like more steps than doing a diesel.
Take care of your two stroke and it wont cut out on you when you need it the most...maybe!
These 2-stroke Mercurys (basically a Tohatsu) are bulletproof engines, but can be pretty stubborn and ornery if they're not used frequently. Ethanol fuel really messes them up on the inside too. The steps outlined above are what I take to make sure my Mercury 5 hp 2-stroke plays nice...and even then, it quits for no reason once or twice a year, marooning me on the dink, usually in the middle of the New River as a 120 ft. megayacht is bearing down on me.
A two-stroke, 9.9 hp mercury is probably around what...100 lbs? Sure it can be done on the boat...but likely not while the engine is in the down position. It would be more comfortable and convenient however, to do the annual service on land. Lot less chance of losing some small, expensive part overboard. And since so many things in these outboards are spring loaded, its very likely you'll lose a part overboard while unscrewing various parts
Its amazing how a teeny tiny idle nozzle being lost can render your entire engine useless/a paperweight.
While it may seem like a lot of work...none of these tasks are particularly hard or time consuming. Just a series of 30 minute projects that when sequenced together can be done in a day. Just watch a lot of YouTube videos on how to do all the various things. Its not rocket science, these engines were designed to be worked on by the backyard mechanic. When you get into the bigger horsepower ranges (25, 35, 100 hp, etc)...thats when I'd get a serious mechanic involved. You could just drop this off to a mechanic to do and he'll probably charge you $300-$500...but for me, thats 30-40% of the the purchase price of a new 5 hp engine!!