i have a 2000 yamaha 50 with a 17 gallon gas tank. how many miles per gallon should i be getting? what is the "average" mpg on a 50hp engine?

Tough to answere man!  Depends on how big your boat is how much its weighed down...Full open throttle, idle.  Water calm? Wavy?

Your best bet is to fill the tank and keep tabs of your own average of miles and hours.

Just my .02

a rule of thumb is a two cycle outboard will use 10% of it's HP in fuel per hour at WOT. So your 50HP will use 5 gallons per hour @ WOT.

go here make a few choices and you should be able to get close to your usage.

http://www.yamaha-motor.com/outboard/products/perfbull.aspx?ls=outboard

here is a 50 on a 17' boat http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_2stroke_hpmidport_al_str-176xt-50tlr.pdf