Was it only those two gauges? No others? First step is make sure all of the connections are good. Either plugged in or screwed to boards. Just follow the lines. Were they working before and stopped or did they never work? Call the previous owner and ask if any work was done prior to you picking it up. After the simple stuff is verified you will have to get a volt meter out and verify where the power is at and where it is not at. Post up a little more info if and when you can. If your not handy, you will be out a couple hundred for a pro at minimum.
It was those two only. From what I can tell everything is hooked up correctly. They worked last year, and when we ran the boat last weekend they were just dead. Its a 8 year old boat, so could the gauges just be worn out and ready to be replaced?
I doubt the gauges went bad after 8 years. Most likely smithing came loose and it will be an easy fix. Does the boat have a computer (ECM)? Is the speed ran from a wheel off the back or a tube? I would troubleshoot the speed first. Probably fix both issues once you find the problem and it should be easier to troubleshoot.
We store it in the garage during the winter so nothing can get to it. our boat is a sterndrive with a mercury. Im not super hand with boats. but I'm assuming that it does not have an ECM. And i do believe that the speed is ran from a tube. Im not 100% sure on that though.
Mercury will have a hole on both sides of the stern drive. If not, there will be a tube next to the drive like the video. 8 year old boat will probably have an ECM. I would bet you will get more info on iboats.com forum. That's where I used to get most of my info from.