When wiring new light to old boat wires how do you tell which is the hot wire on light and from boat?

If you really want to know what you are dealing with, there is no substitute for testing the wire with a multitester or other electrical testing device such as a voltmeter. That's because even when there is a coded wire color that is supposed to tell you what the wire is (traditionally, the white color is the hot wire) people do not always wire things correctly or follow the color coding convention. However, some types of lighting (incandescent bulbs or compact fluorescent lights) will work whichever wires you connect as long as you form a complete circuit. In other Words, the current does not have to flow in a particular direction, as long as it flows. The light will still work. If you have a fluorescent fixture with a ballast that has six wires, then things get more complicated, and you will need your tester.