There are two separate questions hiding inside does sea of thieves have crossplay: whether you can play together, and whether your progress follows you. They often have different answers.
Cross-progression is the rarer feature and the one people actually want. The answer to does sea of thieves have crossplay changes quietly, so anything written more than a season ago is suspect.
Short version
Buy on the platform you actually use most, since purchases almost never transfer even when progress does.
Which platform to buy on
Buying on one platform rarely entitles you to the game on another. Anything marked optional really is optional, so skip it without guilt. Party systems and matchmaking systems can support crossplay independently of each other.
Friend codes work more reliably than platform friend lists for exactly this reason. Store-level differences cause more trouble than platform-level ones.
If someone cannot join the group
Patch notes are dull and they are also the fastest way to find out what changed. The support documentation is the most reliable source here and the least readable one. Invites sometimes only work in one direction, which looks like a bug and is not. Turning crossplay off can shorten queue times or lengthen them dramatically, depending on the platform.
Anti-cheat differences occasionally split platforms even when the publisher intends otherwise. Where a game has a mobile version, it is usually the one that lags behind on updates.
Crossplay versus cross-progression
Treat the numbers as a guideline; your exact result depends on your setup and current version. A shared account is usually the requirement, and creating it later than your friends is a common source of confusion. Console family restrictions sometimes allow one generation and not the other.
Where it breaks
With opt-in crossplay, one account that has it disabled breaks the whole group silently. Progress carried across platforms usually excludes purchased currency, which surprises people. One clean run beats five rushed ones, both for results and for your patience.
- Check the subscription requirement on each platform.
- Wait out the first day after a major update before troubleshooting parity.
- Create and link a publisher account on every platform involved.
- Check the version you are running before following any step-by-step advice.
- Do the boring preparation step, even when it looks skippable.
Avoid these
- Ignoring version mismatch on the day a major update lands.
- Troubleshooting for an hour when one account was never linked.
- Buying a second copy expecting purchases to transfer with the save.
- Blaming the game before checking the one setting that was changed last week.
Questions people also ask
Is there a faster method?
Usually yes, and usually it is riskier. The route above trades a little speed for a much lower chance of having to start over.
Can I turn crossplay off?
Where it is optional, yes, in settings. It usually changes queue times, sometimes considerably.
Is any of this reversible?
Nearly all of it. Anything that is not is flagged in the steps, and there is normally a safer alternative worth considering first.
If a future patch changes something here, the reasoning should still point you at the fix.