Navionics have a nice free online map tool, you can use this free or get a license and add the program to your devices. Take the time to look at your proposed destinations, use the distance calculator to work out how many K;s it might involve and make sure you have enough fuel. (seriously, save yourself the embarrassment) Navionics Site

You will notice a little compass at the bottom right of the map, click it and put the pin in your start and finish point and voila!