Wonder Boy V - Monster World III — Cheat codes

Secrets

Poseidon Ocean Money Farming
After you pass the large strip of land with no ocean, jump in with the Trident, swim one area to the west which will put you below the large strip of land, kill the large fish enemy (Hold up and press B rapidly to spin the trident to kill off the smaller fish it shoots at you as well as damage it), it will either drop a large bag of coins (400-700 Gold), or a large heart which will refill your health. Leave and after killing it, then return to get it to respawn, repeat as much as needed.

Save at any time (Japanese Version only)
If you want to save the game, kill yourself and you will be sent back to the last inn you stayed at without being punished. This is much quicker as you don't have to travel all the way back.

Stay at the Inn for free
Any Inn throughout the game will still let you spend the night, even if you don't have enough gold. If you're short, they will take whatever you have, even if you don't have any gold at all.