Rent a boat
You can rent a boat and sail all over the lake if you want to. I haven´t hired boats myself but I have watched many do it as I sometimes sit at a bench there reading. I don´t know their service but I do know their approx. price. There are two companies on the dock, both having similar boats and renting for about 50 euros an hour, then a small discount for each hour after that. Four hours are 150. Here we are talking about a 50 horsepower boats for up to 6 people. (4 is a better number though.)
If you want a bigger boat other prices are due and you have to have a sailing licence. On <50 horsepower you don´t need a licence. a 4 hour trip on a bigger boat is 200 euros.
All prices are only to give you an idea, they may vary or change and it and don´t blame me if they do:)
I have gone on both a 400horsepower speedboat and on a 36 foot yacht (both privately owned) and it is a dream on a good day do have a lunch, a nap or a swim in the lake. That´s life for you!
You can also hire pedling boats for a few euros. '
There are ferries that go from Desenzano which I haven´t tried but I mean to do so this summer. You can sail either from one place to another, Desenzano-Sirmione, and then back or you can take a longer tour with a short stop at each port. I look forward to it. It isn´t that expensive as I recall it, but I´ll get you more on that subject later.
Added: March 25th 2008:=> I just went with the ferry on lake garda, finally;), and I have one recommendation: There are two kind of ferries, a fast going speedferry and then a slow going normal ferry. If you choose the speedferry, as I did yesterday, you will get very quickly between the towns of Garda, BUT, and for me it is a big but, it is just like sitting in an airplane, you don´t see much and you can´t go outside. I recommend taking the slow ferry. I wanted to take pictures and videos for this page here but I have none as I couldn´t get out and the windows didn´t offer much in case of sightseeing. But it was fast though;) Here is a link to the official Lake Garda Ferry website: http://www.navigazionelaghi.it/eng/gar/nlg.html