There are two choices, you can tell the map for 1,000 adena which I don't think is worth it, or tell him you don't want to sell it.
If you don't sell it he launches into a lot of lore about Captain Zaken. The only important thing to know is you need a Thief Key (I believe it's called
Key of Thief in the Grocery Store) to open the treasure, so don't forget.
2) He marks on your map where the treasure is, and knowing this is only good for this quest, you can't go down there and open it at any other time.
When you are still above water, look for this structure from the beach line.
You may want to either get the Kiss of Eva spell on before you go down, or get the Eva's potions in the Grocery Shop in Heine.
Once you go underwater and get closer to the box you'll see it's surrounded by Great White sharks, should be about 4 of them. Be prepare to take them out.
Then talk to the Pirates Chest, which is under some seaweed.
The rewards vary, all of which should be more than the 1,000 for selling the map outright, but don't forget the cost of the key and porting. This quest also
can give Enchant Armor D and C's, Elven Mithril Gloves and sage's Worn gloves. However in CT2 you can get these cheaply from the shop so it's not the 700k+ prize
it use to be. Still, it's a bit of a fun interactive quest.
Basically there are 4 blocks of rewards out of each of which you can get one or none:
|Musical Scores||Theme of Love|
Theme of Battle
Theme of Comedy
Theme of the Feast
|Greater Dyes||Str+1 Con-3|
|Gloves and Armor Enchant Scrolls||Mithril Gloves|
Elven Mithril Gloves
Sage's Worn Gloves