The seaside walk in Desenzano isn't very wide, but it's still nice. There are benches here and there, so you can sit and enjoy the lovely view. You can also see Sirmione and its castle from here. The walk is lined with big, sharp rocks as well as a small wall. This also means that it's not easy to go for a swim. But I think I saw a beach which you can access easily, but it's not big. It's located next to the "Spiaggia Desenzanino" beach in the central part. This beach isn't a proper beach though. There's also a wall there, but lower at least, but I guess it's more suitable for sunbathing and socialising. There's also a restaurant here.
Also, when it's windy, you have to watch out for big waves that splash over the rocks and onto the seaside walk!! Sitting on a bench under a palm tree and admire the view.