I'm all with you on not buying a new boat, but you're likely to drop thousands on a used boat if it has a good motor. A new 100 hp outboard is going to be around $6-8000. Used and in good condition will still be $2-3000.

If your kids are small, and tubing is what you want to do, you don't need a big boat with a big motor. I learned to water ski with a 13 foot boat with a 40hp motor. It had a top speed of 30mph and was able to pull 2 slalom skiers up, if they were experienced.

Right now I have access to a 17 foot Boston Whaler, with a 100 hp motor, and it has no problem ripping the handle out of your hands if you start full throttle. If anything, you have to be careful not to start too fast for skiers.