Is this the first and only time the garden has been under water? If it's a recurring problem, you'll either need to move the garden to higher ground, or install drainage around, and possibly in the garden, as well as move to a raised bed system. Your rhubarb may be moving into it's dormant period anyway. And, the root system may survive to come back next year. I'd recommend that when the flooding abates, you do some exploratory digging to see if the root system is rotting. If it isn't you should be ok next year. If it is, you may be able to salvage some sound roots, and can do some root cuttings, perhaps even potting them up to get a head start on a rhubarb bed in a more charitable spot next year.