Sir Walter Raleigh states "I set sail from Plymouth in the Spring of 1552". 1552 was around the year he was born.
When Captain Redbeard Rum knocks over the table and says he has no legs you can see a hole in his chair and his legs in it.
Sir Walter Raleigh says he set out from Plymouth in 1552. Queen Elizabeth I didn't become queen until 1558. Had he set out in 1552 he'd be setting out under Queen Mary I.
During Blackadder's conversation with Captain Redbeard Rum, the microphone can be seen several times in the upper right corner.