There are lots of boat trips from Mantova, including trips all the way to Venice. Or you can just take a one hour trip, as I did. See: www.motonaviandes.it
The site shows the schedule for the one hour cruises (8 euros) as leaving at 09.00, 10.00, 11.00, 12.00, 13.00, 14.30, 15.00 and 16.00 and 1.5 hour trips (9 euros) leaving at 10.30, 12.30 and 16.00.
However, when I made the trip (a Tuesday in late June), the only departure on offer was at 15.30 (one hour trip).
Bookings can be made at the Motonavi Andes booking office, Via San Giorgio 2. (Continue in the direction of the waterfront from the entrance to the Palazzo Ducale, the boat office is on your left). Check the opening hours- they close at either 18.00 or 19.00. The boats themselves leave from a landing stage which can be reached from the boat office on Via San Giorgio by continuing down the road away from the entrance to Palazzo Ducale, and when you reach a roundabout turn left along the main Viale Mincio, then cross as soon as you reach a pedestrian crossing and double back, walking down the bank a bit to the waterfront. The boats leave from near to a bar/restaurant. There’s a good map in the Rough Guide. It’s a nice trip- the boat turns right, goes past the Castle, and eventually passes through a narrow channel to briefly enter Lago di Mezzo, before turning back.