Keep in mind that the very surface of the water can get very hot. I'm talking the top few inches of water. Then it drops pretty quick. The top few inches of water can be very sporadic and I have found that temperature checks between 12 and 18" below the surface to be much more consistent and meaningful.

I've used a cheap pool thermometer too and have tied a large nut to it to make it sink and tied knots in the line to determine how deep I'm measuring...18" and 1 foot increments from there. I just hang it off the dock to take my measurements. Without a dock you would have to tie a empty plastic bottle to it at the depth you want to check (think fishing bobber with a weight tied to the thermometer on a longer throw cord).

Some ponds have trouble with aeration and getting murky, some don't. I like to think that a pond is better with aeration, unless it caused the clarity to get less than 18". I would consider starting it for 15 minutes and see it it brings up any muddy water. And, watch for murkier waters as you increase run times.

Fish on!,
Noel