I think 84 would be wishful thinking for the dead of winter. I have a
large bank of aluminum solar panels, in mid florida, in the coolest
part of winter without cover water temp will hit 70 with solar on..but
i'm close to the bay and get alot of breeze flowing across the pool
surface. My solar is good for about a 8 or so degree boost, but
without cover it naturally looses all that at night.

I'd be interested to hear from people with covers and solar in texas
and florida to find out how much heat they save with cover. If it
would hit 80+ I would get a cover in a heart beat. I just dont think @
70, if you get 8 degree boost from solar, then your cover holds it in,
i think at night the shell will still loose too much heat to get even
over 80...but i'm just guessing

let us know what you find out or do.