The regular FitBit Flex is not fully WaterProof!
On it's site, Fitbit states that “Your Flex can be worn while you are in the shower, or working out. It is sweat-proof, rain-proof, water resistant, and can be submerged in water up to ten meters.”
However there is a company that appears to be selling a completely waterproofed version of the FitBit Flex for $199. It's meant for the Fitbit Flex to be used when swimming and makes it waterproof down to 210 feet underwater so you can now log your swimming activity or surf sessions.
You can also pay WaterFi $99 to waterproof yours if you already own a Fitbit Flex. If you can find a good deal on the Fitbit Flex, for example on Amazon for $95 or eBay you can also save some money