Dead after swimming in the sea

@lesly29136 you were right. It supposedly is waterproof up to 50 meters. In the case of your Fitbit, this is now strictly academics since it obviously failed the stated waterproof standard.

You mentioned you were swimming in the sea. I assume that this means we are talking saltwater exposure. In that case time is of the essence since saltware is corrosive. Time to take it apart. For now you want to stop continuing to try to charge, sync or otherwise use your watch. This potentially can make a bad situation worse.This may further damage your watch.

Since iFixit does not yet have a guide for this, use something like this video The next thing to do is to clean it. After you disassembled your Fitbit, remove the EMI shields from the circuit board. then clean the whole board, connectors and all with +90% isopropyl alcohol. Follow this guide. Even so it was written for an Apple iPhone 3G, all the points are still pertinent to your Fitbit. I can not stress the importance of a good cleaning enough, so do it over and over while replacing the alcohol after each cleaning. While you clean your board, check for any obvious damage, like burned or missing components etc. When it is properly cleaned, replace the battery. All these steps are to avoid delayed failure caused by corrosion. Best thing to do would be to clean it with an ultrasonic cleaner, but if you do not have access to one, this will at least help. Once all this is done, reassemble your watch and reevaluate. Until it is cleaned, everything will only be a guess. Even with all this, water damage has only a small chance to be fully repaired, but at least you can give it a try.

While you work on your Fitbit, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!