Frog in or out

I have a 60gal kidney shaped pond.
I clean, put the green frisbee pond heater in, take the pump out and
cover it with half inch square plastic screen griding.

Last year the frog stayed under a rock under the water all winter. This
year will he be alright?
Yesterday, I did the winter close up. The frog was not in the pond when
I started, but when I picked up a flat rock by the rim he was under, he
jumped in and is floating with his eyes and molth near the top, happy as
could be.
Do frogs usually stay under water or on land?
I'ts not a big bull frog, but smaller.