Use your mouse to catch fish. To sell your fish when your boat is full, click on the coin icon in the top left corner. It will give you money depending on how much fish you caught times the fish multiplier (change (coins) by [fish x fishValue]) To buy upgrades, click on the shop icon next to the sell button. You can buy upgrades ranging from some simple fish bait to repairing your engines. [Repairing the engines only makes you sell faster and is pretty much useless.]
This is a solo game (I made it by myself) so I can't really give out many thanks apart from @meeboyay for taking a copy of this game and making it wor- better (see Drunk Fish). See comments made by me to find out what each upgrade does. This game requires quite a bit of patience for you to get a moderate amount of money.