Mate every side has valid arguments, me personally, i would go with a 4 stroke because of some well known issues with a certain cougarcraft that had an etec.....
I have since heard of 4 other etecs where the owners are very unhappy and regretful of there purchase, but i will point out that all of these etecs are big motors 150hp +++.......I have spoken to people like Ross who own smaller etecs and they seem very happy....so maybe the smoke and fuel economy issues are resticted to bigger engines....
I have heard nothing but good things from optimax owners may it be Mariner or Mercury...i know about 5 people with an optimax and they all get excellent economy with good performance.
For offshore fishing nothing but a 4 stroke would be recommended by me, my mate just bought a 610 quintrex with a johnno 4 stroke on board and we only go through 80 litres of fuel after a whole days trolling, and we have found a 4 stroke is much better at maintaining a trolling speed unlike many 2 strokes where you gotta keep playing with the throttle due to the swell.
But you mentioned you may want to tow a skier and if you do i think a 4 stroke would not be suitable, a 4 stroke is slower off the mark and this may effect your water sking capabilities, Me personally, as a fisherman could not give a rats ass if it takes me 2 seconds longer to get to 20 knots as a 4 stroke motor for a offshore fishing boat is awesome and has too many advantages over other motors.
For what you wanna do with it i would recommend a OPtimax motor, good power and performance with great fuel economy....