Red and black In circuit are your 12vdc power and ground, respectively.
Red and black Out circuit will go to your LEDs. Make sure to keep them on the right polarization since they are light emitting Diodes after all. Diodes only pass current in one direction and too much reverse will smoke them.
There are two ways to dim LEDs, one is current limiting and the other is pulse width modulation. If you take a video of your boat with the LED lights on, and they appear to strobe on and off sporadically, that means your dimmer is a PWM dimmer. A current limiting dimmer won't do that. The strobe effect is too fast for your eyes to see but lowers the average light output with the on time ratio is low.