Can't think of a good solution. Maybe the train station, but I'm not even sure whether they have a baggage room there.
No matter what your itinerary, Venice is probably the most interesting place you'll visit. It's a shame to have so little time there, and that you'll be spending most of it trying to stash and retrieve your bags. Plus, if you're flying directly from the States, you're likely to be jet-lagged out of your skull. If it's humanly possible, you should think of changing your plans to arrive two days earlier, enjoy the city, hedge against flight irregularities, kick the jet lag, and solve the bag problem in the bargain.
I know that vacation time is frequently limited, but I always feel badly when I see people rushing through embarkation/debarkation ports like Venice, because it's a far more important and fascinating place than some of the stops on your itinerary at which, ironically, you will have more time.
We don't generally have trouble with jet lag; we sleep well on the plane, with the help of a little Ambien.
Last year our Scandinavian/Baltic cruise started in Stockholm--we landed at 7:30 AM, took the express train into town, stored our luggage in the RR station, enjoyed sightseeing until 3 PM, boarded the ship, took a nap before dinner, and were good to go. I don't think it took us more than 15 minutes total to stash and retrieve our luggage.
We have two days in Venice, which was one reason we selected this cruise, and would like to make the most of them. Everyonbe enjoys a different travel style, and for us, flying in early, checking in and out of a hotel, unpacking and repacking, are not appealing.