The floors climbed are determined by a barometric pressure sensor that is capable of sensing differences in air pressure when elevation changes. Those sensors need to be exposed to the air, as they have a sensitive membrane that converts small air pressure differences to electricity which the watch can then interpret. Unfortunately, those sensors are potentially vulnerable to deviations in the environment they are exposed to including wind, moisture, and dirt.
That all said, you'll find that Fitbit has placed your sensor behind a small hole underneath the band on the bottom of your watch. The sensor needed to be somewhere that wasn't fully sealed off from the external environment, but was mostly sealed off from debris and interference, and that's why they put it there. Unfortunately it's not completely sealed off from water or debris. If it was exposed to water or moisture, it's possible that when fully dry it will return to normal. If some sand or dust got in there blowing a bit of air in there and waiting a bit for the sensor to stabilize may help.
If none of that helps, it's regrettably possible the salt of the ocean water dried on the sensor and altered its properties. It's also possible that the sensor, which is supposedly waterproof, had its o-ring sealing fail, thus damaging the sensor. Contact Fitbit support.
