I suppose that the squirrel was trying to get a drink. I had never seen one drink water before last week and caught sight of one of the neighborhood critters drinking from the pool. Since my dog likes the salty water, I guess the squirrels are attracted too.
I think the squirrel had been drinking from the horse tank in the past and we usually kept it pretty full so for whatever reason, the heat maybe, the water level was low and when she fell in it was too low for her to get out. Thank goodness I went out that day and heard her splashing around.