How To Wire Underwater Boat Lights

Underwater boat lights give your boat a serious wow factor. They are also great for fishing, relaxing, and entertaining. It’s no wonder they are such a popular upgrade.

But what about installation and wiring underwater boat lights? Isn’t it complicated and costly?

That all depends on how many lights you are installing and whether they are single color, multi-color, or RGB lights. For this post, we are going to stick with the basics. I’ll show you how to wire two underwater lights to a switch.

If you need to know how to wire more than two lights then read this post on wiring several underwater boat lights.

Installing Underwater Boat Lights

Before you can wire your lights you first need to install them. Installing underwater boat lights includes several steps.

  • First, decide where you want to mount your lights. The best mounting location is usually on the transom as low in the water as is possible.
  • Next, use a template to check to make sure the lights fit and to mark the location of the power cord hole.
  • Check inside of the boat to make sure there is room to feed the power cord through.
  • If everything looks OK you can drill the power cord hole and mark and drill the screw holes.
  • Finally, mount the lights using silicone caulk and the provided screws.

See this post on how to install underwater boat lights for more information.

Unused Switches

First of all, boat manufacturers often build boats with an unused switch at the helm. They do this because they have a popular accessory, such as underwater lights, that they hope to sell.

Manufacturers build switch panels this way to save money. It’s cheaper to build all switch panels the same way than it is to build two different panels or have a dealer install a switch at a later date.

An unused switch isn’t connected to anything. As a result, nothing happens when you turn it on. However, it is prewired and you can use that to your advantage.

To begin with, a wire will be connected to the out-current side of the switch. As a result, finding where the wire terminates allows you to tap into it to power your lights.

Finding the Right Wire

First of all, the wire will be bundled with other wires and routed to the stern. Your owners manual may have wiring diagrams that show what color wire to look for. The main wire should splice off into two separate wires. It might terminate into a plug.

On boats with outboards, the wires pass through a dry, secure area such as the gunwales. You will often find these hidden behind access panels at the stern.

On the other hand, boats with inboard engines have the wires attached to the forward bulkhead inside the bilge.

Check your owner’s manual or with your dealer to find where to find accessory wires on your boat.

Is This The Right Wire?

The wire will be the same color as the wire coming from the switch. In most cases, the wire color is blue, blue/violet, or red/violet. But it could be another color. Again, check your owner’s manual for a wiring diagram. It will show you what wire color to look for.

Once you find what you think is the right wire, check it with a multimeter. The wire will have power when the switch is on and no power when the switch is off. If it does not, check the fuse block up-current from the switch for a missing fuse.

Once you find the right wire connect your underwater lights directly to it. Connect the red wires from the light to the wires from the switch. Crimp and heat shrink ring connectors to the black wires and connect them to a negative bus bar.

Add a Switch

Many older boats don’t have an unused switch. In that case, you will have to add one. The switch must be marine grade and rated to at least twice the continuous current draw of the lights. For example, an SCM-6 draws 2.1 amps. 2.1 X 2= 4.2 X 2= 8.4 amps. You need a switch rated to at least 10 amps.

The table below shows the different Shadow-Caster single color underwater lights along with the recommended switch and fuse for two lights.

Underwater LightAmps/LightSwitch (2 lights)Fuse (2 lights)

*Lights need to be switched through a 20 amp relay.

Tools You Need

Wire Stripper
Ratcheting Crimping Tool
Heat Gun or Torch

Switch & Fuse

Switch*: The switch you use must be marine grade and rated to at least twice the amperage of the lights connected to it. A single-pole, single-throw switch (SPST) should be used. They are the easiest to wire with only one input terminal and one output terminal. See the chart above for the correct switch amperage for your lights.

Fuse: It is recommended that each light be fused separately. This can be done with an inline fuse rated to at least twice the amperage of the light. However, with only two lights it is OK to use only one fuse to protect both lights.

In this case, the fuse before the switch would protect both lights. Therefore, it must be rated for at least twice the amperage of the two lights combined. See the chart above for the correct fuse.

Wire & Connectors

Wire*: The wire you use must be marine grade. It also must be able to handle twice the amperage of the lights it supplies power to. BlueSea has an excellent app and chart to help you choose the correct wire.

Step-Down Butt Connector*: These are used to connect the power wire from the switch to the two red wires from the lights. They must be marine-grade and sized to match the wires. It is a good practice to use butt connectors with adhesive lined heat shrink insulation.

Ring Connectors*: Ring connectors are used to connect wires to fuse blocks and bus bars. They must be marine grade and sized for the wire they connect to. They should also be sized for the screw that holds them in place. Most of the time these screws are #8-32.

Female Quick Disconnect Terminals*: Quick disconnect terminals attach a wire to the switch terminals. Choose marine grade heat shrink terminals that match the wire size.
*Must Be Marine Grade (tined)

underwater lights for boats
1) Fuse Block
2) Switch
3) Step-Down Butt Connector
4) Single Color Underwater Lights
5) Negative Bus Bar

Wiring Underwater Boat Lights

  1. Mount lights and run wires. Rout the wires through the boat in a way that protects them from standing water, extreme heat, and chafing.
  2. Turn off power at the main battery switch.
  3. Connect fuse block and switch. Cut a length of wire long enough to easily fit between the fuse block and switch. On one end of the wire crimp and heat shrink a ring connector sized correctly for the wire and fuse block screw. On the other end crimp and heat shrink a female quick disconnect. Remove a screw from an empty circuit on the fuse block. Fit the screw through the ring of the connector and tighten it back down. Push quick disconnect terminal onto the inbound pole of the switch.
  4. Run a wire from the switch to light wires. Cut a length of wire long enough to easily reach between the switch and the wires from the lights. On one end crimp and heat shrink a female quick disconnect. On the other end crimp the small end of a step-down butt connector to the wire. Now, place both the red wires from the lights into the large end of the butt connector. Crimp those wires and heat shrink the whole thing.
  5. Crimp and heat shrink correctly sized ring terminals to the black wires from the lights. Rout and connect them to a negative bus bar.
  6. Add a correctly sized fuse to the fuse block fuse.

Wiring More Than 2 Underwater Lights for Boats

Two single color underwater boat lights can be wired using only wire and a few connectors. However, three or more single color lights are much easier to install when using a power distribution box such as the SCM-PD-Relay-4 or SCM-PD. Your wiring will stay neat and dry with fewer problems.

I want to point out that I am not a marine mechanic or electrician. The material in this post is for your information only. All boat wiring should be left up to the experts. You can find a certified marine technician at

I hope you find this information helpful. If you do, please share it with your fellow boaters on your favorite social media website.

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