I found a bug where you can buy an item you haven't unlocked by buying the last item you can afford. I noticed this when I bought the pirate flag and it game me an option to buy the four barreled cannon.
Guys, here's a tip for leveling up: Buy the treasure chest upgrade and do this repeatedly until you get to a really high level, you can get alot of diamonds when you get around to the third upgrade of the treasure chest.
You should be able to grab your coins with your cursor instead of your ship, or atleast make a magnet upgrade. It's a pain when you are fighting countless enemy ships and you risk your way to just get a couple of coins.