Can you use 240V cable for 12V?

Not to confuse you but cable that is usually used on 240v is actually rated at 400v or higher. If you are using 12v and you are worried that the length of run to your water feature will create a voltage drop(most likely will but it will be so small it wont be a problem) then use 1mm2 or 1.5mm2.

What is the difference between 12v and 240V?

12V boats have a cigar lighter socket on board which means that all you are able to charge is your mobile phone in the same way as you would in a car. Some boats have a 240V power supply however. This means that they may have 3 pin sockets on board.