The powerband on two stroke motors is similar regardless of what they look like, outboard, bike, lawnmower etc.
At lower rpms the torque is high compared with the horsepower. As the horsepower increases the torque backs off (relative to the horsepower).
On the 200 Mariner (which is mechanically a 200 Mercury with different paint/decals) the Wide Open Throttle (WOT) range is between 5000 and 5800 rpms. Perfectly tuned, at sea level, with a air temp in the 70 degree range and a relative humidity of 30% it will develop it's full 200 horses at 5400 rpm.
Exceeding 5400 rpms (so say moving up to 5800, even though it's in it's "range") will actually cause a reduction in horsepower of upto 5 to 10%.
When "propping" a particular motor on a particular rig you need to shoot for the mid point of the WOT range - or about 5400 rpms in this particular case.
In order to correctly prop you must first load your boat as you would "normally" use it. Weight is a huge factor in prop selection. "Bob" may have the exact same boat with the exact same motor but his required pitch could be 2 to 4" different than yours.
Additionally, one prop is "NOT" sufficient for the entire season. As humidity and air temperature increases your motor has to work harder. It is not uncommon for a motor that routinely produces 200 horses on a cool spring day to barely be able to achieve 160 horses at mid summer temps. Because of that the prop that would allow you to achieve 5400 rpms in early May is "way too much prop" for the end of July, only allowing the motor to top out at say 4800 rpms, somewhat below it's WOT range.
Even if you don't intend to "run that fast", the motor must at least be capable of reaching it's lower WOT number or it will load up, carbon up, run like crap and could end up becoming damaged.
So, ideally, you should run a minimum of two props. One for cooler weather/lighter running and one for the "dog days" or running heavy.
If that is impractical (for any number of reasons), your prop should be fitted so that it just barely gets you into the WOT range at peak summer (say 5100 rpms max) which should allow you to stay in the upper rpm range (5800'ish) in the spring/fall.
You prop a boat to allow the motor to run properly, any benefit in "gas mileage" is incidental - you shouldn't mess with prop selection to try and increase gas mileage if it allowing the motor to run as designed.
Better gas mileage can be achieved through numerous other factors (but I am talking about small numbers here - anything that promises 10% better gas mileage is better used to top off the pile behind the "horses barn", for that's where it belongs).
Installing trim tabs can improve your gas mileage a couple percent. Likewise, mounting the motor on a jackplate - and you don't have to go extreme, an adjustable with 4 or 6" of setback will allow you to fine tune the motor's mounting height which will allow you to squeeze out an extra 1/2 gallon per hour on this size motor. Keeping the bottom clean and keeping the weight properly distributed will likewise improve fuel economy.
But at the end of the day this is still a 200 horse motor. Running it wide open will result in burning 20 gallons per hour. Running at it's "cruise speed" which is about 3/4 throttle (3600'ish rpms) will still cause it to burn 13 - 14 gallons per hour (not cheap to run), but over a season, if you use it alot, squeezing out an extra 1/2 to 1 gph can add up to alot of money.
So do prop it according to the rpms - shoot for 5400'ish, and then stay easy on the throttle if you want to save gas...
