Ponds and Water Gardens - Most reliable pump?

This is not really a pond or watergarden question, but is a general question about pond pumps.
I've been looking at pond pumps to use as a water circulator to prevent water from freezing around my small wooden dock.
Earlier this year I bought a standard 1/3 hp Utility pump or sump pump that was rated at around 2300 gph. from the local hardware store. I think it was a Simmer. I hung it from a rope about 3 or 4 feet below the water surface under my dock and put it on a timer for 6 hours on/6 hours off. It worked perfectly at keeping the water from freezing around the dock until it broke down. I don't think those utility pumps are meant for such continuous opperation.

I've been looking at pond pumps to use, and since they are meant for more continuous operation like this. I just need to "push" warmer water from 3 or 4 feet below the surface up to the surface to prevent freezing.
Has anyone ever heard of someone using a pond pump for this purpose?

I've been looking at one particular pump "Pondmaster Skimmer/Waterfall pump 3000 gph". It says it is designed for continuous operation and is rated for more gph than the pump that I had previously used.

What do you think about my idea? and my choice of pumps?

Thanks for your help!

to keep ice from forming around the boat,,, if you just HAVE to use a pump, try zoeller.

If you haven't already gotten your pump and you want one Aquascapes pumps are great low amp draw and longest warranty 2 yrs.

I've already purchased a pump.
I went with the Pondmaster Pro Hy-Drive Pump - 4000 gph

http://www.pondmasterstore.com/home/pm1/page_163_53/pro_hy-drive_4000.html

We shall see how it works, I liked it because it is oil-less. and Mag drive, yet still rated for high head hights.