Will saltwater damage my Apple Watch?

Salt water per se won't damage an Watch. But as the k_p95 linked water resistance article points out, it's what may have happened to your watch before you went into the ocean that would determine if the water would damage/destroy your watch. If any prior event damaged the watch's seals, then submerging the watch in any water, salt or fresh, could damage or destroy the watch.

Since you state the watch gets warm on the charging puck, then as Jeff Donald suggested, you should have a professional examine the watch.

M1 Mac mini, macOS 14.5;  Watch, series 7 GPS/cellular, watchOS 10.5; iPhone 12 Pro, iOS 17.5; TV 4th Gen, tvOS 17.5; HomePods, audioOS 17.5 and Airport Express w/ Airplay 2

Approximately how deep was your Watch? Rinse watch with fresh water and dry thoroughly. Once dry, try turning it on. If battery needs charging, place on Apple charging puck/cable and observe carefully for 15 to 20 minutes. If watch get very warm or hot discontinue charging and remove charger from wall outlet. Take watch to Apple for diagnostics and possible repair.

Probably a little moisture increased resistance in the charging system and components heated up. Letting is dry completely has lowered resistance and it back to normal.

I hope your watch continues to serve you well and you have a great weekend!

Hi,

I would recommend following the steps in this article About Apple Watch water resistance. Hope this helps.

Does the screen come on?

At this point, I would be leary of ever wearing it while swimming again.

The screen comes on, but the Watch got hot charging? Are you using an Apple brand cable/puck and wall charger?