http://www.boatliftwarehouse.com/boat_lifts/pwc_lofts/....--} click on "PWC lifts".....
This website has many different types of lifts, it might help give you an idea and you can look at some pictures of how they are built. There is another forum that talks about seadoo lifts you can check out. I just looked at your photo, and whatever you put in the water should not rust and has to be the proper materials.
You can buy the winch. The metal frame is a lot better than wood I'm assuming for this project, so idk I'd you have to weld it like the photo or not. Best of luck, you might find some answers on Google where others have done it. On you tube many people have drive up ramps, no winch they made using a wood frame, carpet or plastic rails, etc. Best of luck!