Besides the water temp and probable viz, why go south for a cleanup dive-- there are likely opportunities close to home!
Our local dive club does clean-up dives in the Windsor, Ontario area on occasion. Usually done at local beaches or popular boating destinations around Earth Day (before the recreational boaters get out).
Altough I wasn't able to make the 2007 cleanup dive, the one I did do was a load of fun. The amount of garbage brought up is really quite amazing (tires, bottles, beer cans, etc. I even retreived a rusty old gas barbeque).