How can I find Marin?
Solution 1:
If you refer to dungeon 8 "Turtle Rock" with saying you opened the last dungeon, you should just finish the dungeon. For me she was back in Mabe village after finishing the dungeon.
I wanted to talk to her to say my last good bye before waking the Windfish, so i am sure about it.
The Pineapple is for the trade sequence of the game, which does contribute to the story as well but is not needed until the very end.
The last Item you get from the trade sequence will help you find the code you need to traverse the windfish egg.