Another thing to be aware of is some repair shops are required to report to CarFax. You might get it repaired but when you go to sell it it will carry a 'flood damage' report on it and NO one will buy it and/or let you trade it in.
Which is WHY you need to have your insurance total it. Bring that to their attention which BTW they are well aware of that also. That should change their mind quite quickly.
Lot in the news of flood damage vehicles being fixed and sold to the unsuspecting buyer so the media coverage is a plus for you at this time regarding getting the insurance company to total it. They won't want to be anywhere near being a part of that.
Just stay on your insurance company to total it. The squeaky wheel gets the grease. They hold out until you keep asking and then they will do it.