Since the boat movement is mandatory on every turn. Let's say that I begin my turn adjacent to the exact tile that I want and after my one movement, I no longer have access to it. Does the rule about paying also apply to a tile now behind the ship after the movement? Or is it only for tiles ahead ? Can someone let me know? Thanks
There is nothing you can do about tiles ahead, nor about tiles behind. You can only purchase tiles next to the exact row or column where you end your move. For anything else it is as you already said, "I no longer have access to it".
An extended quote from the rules (with my highlight)
2. Discover a tile
After the player moves the discoverer ship, the player must do one of the following:
• Take 1 tile from the matrix and place it on his board.
• Take 1 tile from the matrix and place it in his storage.
• Take 1 tile from his storage and place it on his board. • Discard 1 tile from his board.
• Do nothing.
•Take one tile from the matrix and place it on his board
This is the normal action and the most frequent executed. – He takes the edge tile next to the ship free of charge, or takes another tile from the row or column. For each tile skipped, he pays 1 of his shells to the supply.
The rules continue with a picture of a row where the ship movement has ended and demonstrate how you can purchase deeper into the matrix - only for that row.