Karen,
I am in Tucson, AZ and have a 5500 gallon pond that used to be a 14x28' swimming pool so my situation is not too different than yours. I swear by the Oase Pondvac 3. Some say it doesn't do a good job in 3' depth but ours is slightly over 3 feet and works great. The benefit of a pondvac 3 is that it has two tanks. When one tank is full it automatically switches to the other tank. While the first tank empties, the second tank fills, then they switch again. You attach a pool vacuum (or regular vacuum) type hose to the outlet and direct the used water wherever you want it. The pondvac comes with all needed hoses, nozzles, etc. The vacuum only stops sucking for a second at the moment of the switch and you never have to stop to manually empty the tank like you would a shop vac. The pond vac 3 is not cheap but if ours were to break we would replace it in an instant. We have had it for about 3 years and have had no problems. I sucks up the pond mulm, dead plant parts, fish poop, small rocks, etc.
Casey