The reward inside the bottles will usually depend on their colour. Green bottles generally reward you with some coins and wood while the pink shiny bottles are more valuable as they are likely to have shells in them. Shells is the premium currency of the game.
Double Bait
You will sometimes be rewarded the double bait which you should take advantage of at rare fishing spots to grab as much wood, coins and shells as you can. The only other way to get the double bait is to buy some with real money.
Finding Shells
There are several ways you can earn shells, you can earn them from finding shiny bottles, completing daily missions, and the Weel of Fortune. It is also possible to find shells at rare fishing spots, these will be indicated by water droplets flying out of the water.
Fixing Fishing Rod
Once the durability of your cat's fishing rod hits zero percent it will break. When this happens you can either wait a couple of hours for it to repair itself or you can pay shells to instantly get it fixed. Shells are rare so it is recommended that you wait for it to repair itself.
Open Bottles
When the game starts make sure you keep opening bottles as quickly as possible until you have filled up your three slots. Once a bottle has been grabbed it will start to open and it's timer will automatically begin.
Pink Shiny Bottles
Whenever you see a pink shiny bottle floating in the sea you need to consider clearing space for it as it is likely to contain shells and being the premium currency of the game they are hard to come by.
Rare Fishing Spots
Make sure whenever a rare fishing spot spawns you go there as it is likely to yeild plendy of wood, coins, and sometimes shells. Rare fishing spots can be identified by their bubbly surface.
Shells
Make sure you hang onto shells for unlocking new cats. When a cat box is delivered to you through Ameowzon you will have to pay a shell fee to open it.
Upgrade
Make sure you upgrade your boat whenever possible, everytime you do so you will gain one more slot for another cat which will enable you to earn more coins automatically over time. Upgrading your boat is not easy as it requires a lot of coins and wood.
Colour Bottles...