Apple Watch in salty water

You can use your Apple Watch in saltwater but it is recommended to rinse it off with fresh water afterwards.

Clean Apple Watch if it comes in contact with anything that may cause stains or other damage, such as dirt or sand, makeup, ink, soap, detergent, acids or acidic foods, or comes in contact with liquids other than fresh water, including those that may lead to skin irritation such as sweat, salt water, soapy water, pool water, perfume, insect repellent, lotions, sunscreen, oil, adhesive remover, hair dye, or solvents.

See this link for further details: Important handling information for Apple Watch - Apple Support

Yes see here;

Swim with your Apple Watch – Apple Support

As long as your watch is not damaged yes you can.

I do with mine.