Unfortunately for some, there are no official cheats for Boom Beach. The developers of the game are very generous in making sure that there are plenty of Diamonds to go around, and unlike lots of other free games out there, you shouldn't have too much trouble amassing a small fortune in the premium currency.
Most similar games allow you to speed-up production by changing the system clock, and while this appears to work in Boom Beach, you will get an error instead of a completed building. For advice on how to get free Diamonds, you are best to simply explore the game map, looking for the treasure chests that you will find dotted around. You should try not to spend too many Diamonds on instantly finishing upgrades and production if you can help it - the best times to use it are when you want to be on the game all day - but if you are about to go away it's a good idea to save any long upgrade jobs like the HQ levels for this time.
Of course, the big frustration is that you can only upgrade one item at a time, so if you have your heart set on a particular unit or upgrade, you will need to spend Diamonds to be able to get it done quickly. If you see adverts or users claiming to be able to allow you to get infinite Diamonds, or as many as you want for free, beware. This is usually a con and you should certainly never give out your username or password to anyone. Any downloads from third party sites claiming to allow you to put cheats on your game are also running the risk of compromising the security of your device and your accounts.
The game puts lots of tresure chets in the map for you to find to satisfy most Diamond-hungry players