Question: as far as the coastline goes one bohemian king held control over Poland or part of Poland that touched the Baltic, if I'm remembering correctly from my Czech studies. Did his control over it vanish when he died, and if so, why?

The location of Bohemia was well known enough to a well-read and knowledgeable person of the time that Ben Jonson, a well-read knowledgeable person of the time, noted it as one of Shakespeare's errors. In Jonson's words: ''Shakespeare in a play brought in a number of men saying that they had suffered shipwreck in Bohemia, where there is no sea by some hundred miles.'' Ben Jonson was of the opinion that Shakespeare's lack of education (no Greek and poor Latin, in his estimation) showed in his work. And that this is but one example of a mistake a more knowledgeable man wouldn't have made.

As has been suggested, Shakespeare got essentially the entire plot of A Winter's Tale, including the coastal tidbit, from Robert Greene's Pandosto. (From Greene's work "But upon the fourth day about ten of the clock, the wind began to cease, the sea to wax calm, and the sky to be clear, and the Mariners descried the coast of Bohemia, shooting of their ordinance for joy that they had escaped such a fearful tempest.") It wasn't Shakespeare's error. But he either didn't know enough to fix it or knew but didn't think it important enough to fix.

There are some who say it wasn't an error at all. Either stating that the ancient kingdom of Bohemia had a coast (as argued by George Lyman Kittredge) or that there was a brief interval where land acquisition could be said to give "Bohemia" a coast in more recent history if you use a generous definition of Bohemia.

I think it's a stretch to claim that was on Shakespeare's mind. Robert Greene was highly educated (Cambridge) and it may very well have been on his mind when he wrote it.

Robert Greene is largely credited with writing a pamphlet against Shakespeare and other uneducated writers who think they can write as well as university educated men. (Although this was early in his career and largely concerned with his histories.) So, it's a bit of irony that one of the errors credited to the uneducated Shakespeare may have come from Greene, himself.

To answer your question. Yes, it's something that a knowledgeable and well-read Englishman from that time and place likely should have known. This is evidenced by Jonson expecting others of his standing to understand why this is such a silly mistake. Shakespeare was self-educated (except for his likely stint in grammar school, which would have taught him Latin, which was very important but not pertinent to geography, in this case) so it's possible he just didn't know. It's also possible he knew quite well but didn't see a reason to change the original as it fit the story better. And he knew the masses of the theater would likely think nothing of it.