In no particular order:

Marine mammals

Sunsets with no land in sight. Sunsets framing the land. Sunrises with no land in sight. Sunrises framing the land. Thunder and lightning rolling up one side of an island in the distance, putting on a show.

Dark skies and the stars.

The peace and tranquility of quiet places with just nature and you. Until the sod over the hill turns on their generator.

Fresh fish.

The feel of sea spray, wind in the ears, the connection to your boat, knowing if things are right just by the feel and sound.

And that's just from doing non-ocean-crossing sailing.