I lost my Garmin Fenix 6x in the sea

Garmin's official response - Can Garmin Do Anything About My Lost or Stolen Device?

Otherwise, might need Robert Ballard.  

Unfortunately, RF doesn't really work underwater. This means the watch can't receive GPS signal, nor can it broadcast Bluetooth or anything else more than a couple feet, at most. So you would really need to know where it is within a small enough radius that you can physically see it, or at least get your (very waterproof) phone within 1-2 feet of it.

Depends on how deep, what kind of sea bed. Snorkeling at day or at night with flashlight or renting a waterproof metal detector. That is unlikely to be succesfull, but snorkeling might be worth a try if you now the location quite presicely.

You can look for the phone using the watch, but there is no option to look for the watch using the phone. Maybe there is an app for the phone which sends notifications all the time - so if the watch has BT enabled and is in the range it will beep/vibrate making it easier to find. Any way you will need a diver to look for the watch. And above method (sending notifications) may require the phone to be on the diver (BT range is not that good on land, I have no idea how it is under water).

Anyway, for future reference, 0.02$ solution for 700$ problem.

iLikePotatoes said:

but there is no option to look for the watch using the phone

Sure there is:

"Find My fenix" on the device page of the GC phone app

Bluetooth doesn't work AT ALL underwater, so if you're thinking or hoping for something in that area - you'll need to come up with a new plan. All the various 'FindMe' functions typically depend on Bluetooth, so -  - that's not going to work. Perhaps some other method....[but what that could be, I have NO clue....]

If you by any chance have morning alarm set, you might hear it if you dive at the time it goes off (or see the light if it is still dark at that hour). High pitch sounds travel really well underwater. But you still need to be near, maybe 10-20 meter radius from it.

That's terrible! I don't think there is any substantial part of a 6X that is ferromagnetic, otherwise magnet fishing might be an option, if you knew the exact spot and it wasn't too deep. What type of band were you using?

3928202 said:

But you still need to be near, maybe 10-20 meter radius from it.

You really think you could hear an alarm from your Fenix 10-20 meters (32-64 feet) away underwater?  Seriously?

Potentially, in silent conditions. There are factors involved like better impedance matching in water and superior propagation, sounds are really load underwater, you might expect +30-50dB. Haven't tried with fenix, speaking only from diving experience and some knowledge of hydroaccoustics and just trying to help the poor guy with ideas.