The first thing you see when you drop into the Sea of Thieves isn’t the sea; it’s a table, a grubby, worn, wooden thing in a dingy tavern, dimly lit by candles, the hurdy-gurdy strains of some shanty (or Ride of the Valkyries) warbling in the air. There’s a woman at the bar who’ll offer bits of gossip or refill your tankard (for free - the business model is unclear).
Through the door, out into the too-bright sun, and you’re greeted by shanty lean-to stores and tents - where you’ll pick up Voyages, and (eventually) buy new boots, hats, weapons, tools - all looking like they’ve been cobbled together from the remains of whatever ships washed up here before you did. A short distance away, bobbing on one of the best water simulations you’ve ever seen, is your ship. Part vehicle, part home base, she’s your Falcon, your Serenity, your Black Pearl.
As a great captain once said, what a ship is - is freedom.