That's a great point, except I(and the director) used the term "historical fantasy". They weren't "super interested" in realism, just like Star Wars wasn't. They(for the first three movies anyway) went to locations and used little touches of realism to add to the "feel" of the movie. 

Also the undead pirates weren't the ones who could summon the Kraken. That was Davy Jones in the second movie.