The Fitbit Versa 2 should be waterproof to a depth of 50 metres.
How long have you had the device?
According to the warranty there is a 1 year (or 2 year depending on your location) warranty on it. If it is still within the warranty period for your location, make a claim for a warranty repair or replacement.
If no longer under warranty, do not try to charge or to turn on your device and then remove the battery as soon as possible from the device to minimize any further damage.
The impurities in the water causes corrosion and provides circuit paths for the electricity which were not in the device's operating design and could damage the components. The corrosion starts immediately and is ongoing until it is completely cleaned away
Then you need to dis-assemble the rest of the device and clean all the affected parts using Isopropyl Alcohol 99%+ (available at electronics parts stores) to remove all traces of corrosion and water. If possible do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, can contain additives which may leave residues and is not as effective. If you do have to use it check the label to verify the amount of IPA. The higher the percentage of IPA the better
Here is a link that describes the process. Electronics Water Damage
Here's a teardown video that may help.
As for the other devices, here's a link to just one parts supplier if you need any replacement parts. There are other suppliers online that may suit you better. Just search for Fitbit Versa 2 parts to get results The teardown guide above may help with dis-assembly etc.