I currently own two supercharged models - a 2010 GTX Limited iS 260 and a new 2012 RXP-X 260 RS.
You should consider the ongoing maintenance and fuel costs of supercharged vs non-supercharged.
The older supercharged models need a complete supercharger overhaul at 100 hours but the newer models only need this at 200 hours.
You need to assess whether non-supercharged would satisfactorily meet your requirements - if so, you will save a considerable amount on maintenance and fuel.
I find that I only drive these supercharged models in 'sports' mode (eco or touring mode feels very sluggish after sports mode) but I pay for this pleasure at the fuel bowser with 98 octance costing $A1.56. per litre ($US7.38 per gallon) here in AUS (I have emptied these supercharged models easily in a half day of 'fun').
Re using a ski for wakeboarding - although the ski will tow OK I don't really think that the ski wake is anywhere near comparable to that of a dedicated wakeboard boat - regardless of water ballast, etc. To do any type of towing with a ski here in AUS requires a three seater - driver, observer and person to be towed (unless you start the tow from the shore each time - difficult).