98-99 SK's had twin 720's where the plain Speedster had the twin 787's. Supposedly the SK's with the 720's were at a better power range for skiing--hence they were the SK models. I like the 787's for skiing, power is at a different range so generally I'll ski around 4500+/- RPM, but weighing 230# that's not a problem for me. For others, I just tell them to hold on and enjoy the ride, let go when your tired!
The M2 is the Merc Jet set up, a single jet tied to a Merc 240--one engine, one jet. For some a nice set up, I like my twins so I can limp home on one if needed, I'm not much of a rower if you know what I mean.