There is nothing much to do yet. Tetra wakes up at the sea bottom, in Hyrule Castle's courtyard. The voice tells Link and Tetra to come to where Link got his Master Sword. So go there and hopefully some plot points can be resolved.
Long story short, the king is the voice of the boat! Another point is Tetra necklace with the parted Triforce came together with the part the king hold and transforms her to Princess Zelda! A trademark in Zelda games as she plays an important part in helping Link with his quest. The king recommends Tetra now Zelda stays where the Master Sword is.
Lastly the king tells Link to go up to the surface, now the situations changed to combine all the elements to bring the spark back to his Master Sword.
Note: You can choose to do the Temples with the companions first to bring the power back to the sword or go on a long sailing quest to find all the shards of the Triforce of Courage. You have no limits on what you can and can't do in this open ended world of water.