I live there and I can answer all of your questions ;)
1) Canyon lake is a wide open lake with only a couple of decent spots (West side near the dam and East side in the tributary). During the week, you should be able to find water, but on the weekend your screwed. Better water is in nearby New Braunfels (where schlitterbaun is) on Lake Dunlop. Its just a wide spot in the Guadalupe river a couple of miles long, but it has much much better water and very few people go there (compared to Canyon). The public boat ramp is just under I35 between Hwy 725 and Hwy 46.
2) Texas Ski Ranch (the cable park) is about 5 minutes north on I35 from New Braunfels. If you bring all your own gear (including a helmet) a 2hr session is about $20 (though they have never asked me to leave when I am over my time). The best time to go (weekend) is first thing in the morning. In the afternoon it gets hot and all the kids start showing up. Night sessions last until midnight (I think) and are good fun. During the week, anytime before 5 is a good bet.
3) Schlitterbaun... don't waste your money. Is $32 for an adult to get in the door. If you want to toob the river (Schlitterbaun sits on the river and dumps several rides into it), then just rent a tube (for $5) and ride. You'll have more fun taking the girlfriend and her family wakeboarding than going to an overpriced waterpark with 1000's of screaming kids. BUT, they do have some cool H2O roller costers that shoot you UP hill.. not worth $32 (or waiting in line), but still kinda fun. Be there when the doors open so you don't wait 30min per ride... even on a weekday.
If you bring your boat and have a spot for an extra rider, id be glad to show you where the boat ramp is exactly at