I'm not sure a solar cover would support the weight of a 70 lb dog. And if he goes in with a cover wrapped around him he'd be even more likely to drown.
If you can't fence it, your best bet is to make sure the dog can swim (as in spend a lot of time taking him swimming) AND teach him the way out (assuming there are stairs, and if not, put some in). Some people suggest marking the area where the stairs is with some object -- a flag or planter or something -- that the dog can locate and swim to if he needs to get out.