Defeat the final Boss and save your game after the credits. Reload the cleared saved game file, then sail south from Tonfon. You will eventually find a "C" shaped peninsula at the edge of the world. Enter the whirlpool at that location then sail east from the exit. You will find an island with a cave that leads to the entrance of Crossbone Isle.
Easy experience
When you are at Apollo Sanctum and activating the device, after the cinematic where the men fall from the airship, run around getting into battles. The enemies have around 500 HP each and deal very little damage. If Matthew has the Sol Blade equipped, he may be able to take down enemies with one hit. You can easily gain over 21,000 to 45,000 experience points per battle, along with a Water of Life.
Easy money
At the end of the game, before you battle the Chaos Chimera, walk around the Apollo Sanctum to get into random battles with the Tua Warriors. It is recommended that you have one of your party members, that is not in your "battle party" equip the Lure Cap. They will almost always drop Water of Lifes. You can sell them for 2250 coins. Thirty of them equals up to 67,500 coins. 240 of them (your entire party) would equal up to 540,000 coins.
Ghost Ship
Defeat the final Boss and save your game after the credits. Reload the cleared saved game file, then sail north of Haran island until you find some rocks shaped like a hand appearing from the water. Sail against the middle rock, then sail left into the purple fog until you reach the Ghost Ship. The Ghost Ship is where you can find the summon Azul, guarded by the Star Magician.
Crossbone Isle dungeon...