You choose a trainer by talking those green totems that came from under ground. Kenneth is probably at the camp with the other if you talket to him and told his opponent is the captain, and would like to duel him.
The sailor cannot really be missed if you search the island, there is a small cutscene when you come close enough.
Hey there. I did this part of the game last week, the sailor is near the main entrance to the battle tower. It's to the right of the first set of giant whale bones you come across...atleast i think it is the first set. You should see some ruins (broken columns)and a group of dragon elves. Hope that helps and that i didn't give too much away.