Re: Question from a begginer!!!!
Smallest motor I have ever successfully skied behind was a 70HP, smallest I have ever successfully kneeboarded behind was 40HP (with only one person in the boat).
I don't think you'll be doing much of either of those behind a 25....
Now a tube is different, it really doesnt take a whole lot to pull a tube... (you won't go really fast), but at least its something... (A tube floats and supports most of the weight of the rider on its own, versus a ski or kneeboard where it has to get the rider up on plane).
I have even pulled a tube with my jon boat and 15hp 4-stroke honda... not fast mind you, but my 5yo loves it hehe.
Like Chris said, its better than nothing... but if you had to pick what items to bring, and gear up for, I would try for the tube first, then maybe mess around with the others when you had time to experiment.