Break it in properly,decarb from time to time, &
They love to rev and be blown out. After break in dont be afraid to rev to motor into the high rpms even WOT for a few seconds from time to time.
Get a Racor gas/water seperator with the clear plastic bowl, empty water when you see some.
My opinion on flushing is different then most peoples I think but I think its not a good idea. It screws up waterpumps, overheats the engine @ times and I think the anti corrisive paints & alloys of today are so good that you'll put 5000 hrs. on it before its going to give you issues from not being flushed{ also it's a pain in the a**}
Mercs , OMC'c are good motors some of them seems you cant kill BUY as someone who's put many hours on Outboards next time buy a Yamaha as I have last motor & am blown away by its quality.
or a Honda 4 stroke. Suzuki pretty good too.
Change the water pump every 2 years or 125 hrs. or so, gear oil every 100 @ least. , grease the motor well, spline too so prop doesnt salt on.
Uce Seafoam, Ringfree of Carbon Guard in the gas every other tankfull.
Alan