Each time you reach a port, play the spinning wheel mini-game. You can win Black Stones or even Gems. Black Stones are used to get new skills for your Captain.
Money
When you are still trying to save coins, try selling your fish when the market price is better than the average. This is indicated when the arrow near the fish points up and turns blue. There is a man at the upper left side that always has a fish selling mission. Deliver the desired number and type of fish to him and he will pay twice the market value. Always try to complete his requests. Sell all the fish that he does not want and save the ones he needs until you have enough to sell them to him. You can even wait for the market value to be above the average so that you get even more money from him.
Monsters
You will occasionally get an option to fight a monster and put it into a jail. The monster will award you with resources over time, depending on its level. The jail starts with only three monster cells. You must save 1.2 million coins to expand it further. However you can sell the monsters that have been captured to make room for new monsters. For example, the 1 star monster is worth 100,000 coins when sold. Consider selling monsters to keep one cage available when a new monster appears. Although you might get less low level materials, you will earn money faster.
Reincarnation
Reincarnation will reset almost everything you already have purchased, except for your Captain's skills. However the process will return Black Stones based on the level of your ship. The rewards increase with each new level. Consider reincarnating the first when your ship is level 8. Then reincarnate again after reaching level 9, and repeat the process in the same manner, waiting until you are one level higher than the last time you did the process.
Resources
You will need more advanced materials to upgrade your ship and crew as the game progresses. As soon as higher tiers of resources are required, consider using the older resources to create 50 of the new type of resource then sell the remaining stock to save inventory space.
Scoring
Tap the fish or material icons above your fishermen whenever possible. This will make fishing faster and increase the fishing frenzy gauge. When full, you can fish continuously for more money, fish and resources. The game does not progress while the app is closed. Consider keeping it running whenever your device is plugged in and sell your fish at regular intervals to keep making a constant profit.
Skills
There are eleven skills you can purchase and upgrade for your captain. They expensive and some are more valuable than others. Start with skills that are immediately useful, such as Leadership (faster speed for more mini games), Charm (more money), and Charisma. Save skills that least useful, such as those related to monsters and materials, until later.
Black Stones...