I haven't converted boat lights, but on a friends truck, the LED's have the same number for bulbs. He replaced his 3157 tail light bulbs to 3157 LED bulbs.
The only thing is that you will have to wire in resistors, because the LED's draw less power. If you just plug in an LED into the socket without a resistor, your blinkers will flash at twice the speed. (Obvious and annoying).
So, you can do it without any problems if you wire in a resistor on each light. There are lots of articles and website selling kits, so that's also something you may look into, especially for boat lights. :mrgreen: