The side nav lights are fed from the same switch so if one works that means the switch and both the 12 volts feed as well as the ground is good at least to the light that works. The other side nav light is teed off of the same wires as the working light. What is not obvious is which light feeds the other but in general the nav light closest to the helm (the starboard light) is fed from the switch and then breiged over to the other light.
If the light doesn't work, it means that there is no 12V, or a bad ground, or a bad bulb, or a bad fixture. You can sort of reach the wires in the cabin by feeling all the way under the gunnels where the wires may be run.
It's hard to say what the next step should be but I would probably remove the fixture that doesn't work and try to pull out the wires until you get to where the light pig tails (wires) are spliced onto the feeds. Those connections may be bad. All of this assumes one lamp works. If not you have to test starting at the switch panel which you will have to carefully remove and gently pull away to get to the back of it. If you don't have any experience tracing circuits to test for voltage or good grounds, it would help to see if a boater friend who has tracked down DC issues can offer their advice and guidance.
Do me a favor first though; If one lamp works, remove the lens on that light and remove the bulb. Now install the bad bulb in the good fixture and the good bulb in the bad fixture. See what happens. If the same side lights up, both bulbs are good. If the bad side now works but the previous side that works, not doesn't, that bulb is bad.
Let us know how things work out.