If you go north from izoold without doing two things to get on the boat it will cause the boat to disappear. However, you are not stuck. You will find the girl again, but you must do other things first. This is also an opportunity to get Sheena. This will require a lot of leveling up, but is worth it. There are also two seals that can be opened. You must go to Himma and tell the woman that you are Sheena's friend. This will get you started towards getting Sheena. Luin will be attacked by the Desians and you must save it. You must save the girl being sacrificed to the fake Wind Guardian. Then, open that seal. You can also go through the Tower Of Mana and release a seal. After doing everything that you can, you should have Sheena in your party and the boat and Lyla will be in Izoold, ready for you. You much more powerful than you actually need to be to do everything needed in Palmacosta and the surrounding areas.