You don't really have a Mariner - that one is just a 9.9 Merc painted grey with different decals.
That model began production in 1986 and continued until about 2005 in the US and is still in production (overseas) and is currently sold in Canada (amoung other places outside the US). There was some minor changes made part way through the 1994 model year but about 98% of the motor remained unchanged.
The powerhead was Merc's "split block", also referred to as a "jug head". It (the powerhead) and the gears were mfg by Yamaha (actually a Yamaha company called Sanshin which was also partly owned by Brunswick the company that owns Merc). These powerheads, which were used on the 6 thru 25 horse Merc and Merc/Mariner models was probably the best small powerhead ever produced.
Merc coupled it with their Thunderbolt 4 ignition and a Walbro carb to produce some of the best outboards ever seen.
I own three motors from this (family), a 1986, 1987 and 1991. Two of them have well over 1000 hours of "actual" run time and the baby is just into the 700 hour range. It is pretty difficult to kill one of these and parts are available from multiple sources.
Since your motor is now 19 years old (20 model years) it probably only has 30 or 40 more years before it needs a good overhaul - of course, that's assuming you keep up with the regular maintenance.
There is a couple of issues that due arise with these as they age. The upper crank seal wears out (it's a combination metal, rubber, spring thingy that cinches around the crank right beneath the flywheel - but it's cheap and an easy change). Likewise, the intake gasket (the one on the side of the cylinders) wears out (ditto - 5 minute job). Both of these conditions first become apparent by 1) the motor becomes hard to start and runs like crap until you give it some throttle and 2) it looks like you have sprayed the powerhead with fogging oil (starboard side if it's the gasket, from under the flywheel if it's the seal).
One other issue that seems to be more prevalent in northern climates (like mine where you get cold winters/hot summers) is the original coils develop hairline cracks - so it's worth giving them a wipe down once a year (after they are about 20 years old) and giving them a really good look over - again though, pretty easy fix and doesn't empty the wallet if you have to replace one (Sierra part at about 20 bucks).
All in all you can not find a better 9.9 than a Merc or Merc/Mariner 2 stroke...
. I have never owned a Mariner, how do they rate on reliability, etc.?