Swimming
This is generally covered, as it's considered low risk by insurers.
Snorkelling
As it's usually done in shallow waters, snorkelling is often covered because it poses minimal risk.
Stand-up paddleboarding
This popular watersports is often included in most standard travel insurance policies.
Sailing
This is usually covered. But, if you're planning to race or travel far from shore, you might need extra coverage.
Bodyboarding
This is considered a relatively safe activity, so it's often covered by standard policies.