This happens many times throughout the game. We get a quest (like the ones from our teachers) to go to an area that we have not opened yet.
Continue to follow the Main Storyline quests, and eventually you will gain access to take the boat over to The Krokosphinx.
First you must complete the main storyline quests in The Pyramid of The Sun. This link shows the three main locations in Krokotopia, which are The Pyramid of The Sun, The Krokosphinx, and The Tomb of Storms: