Did you check how much voltage you're getting when you take a reading across the ignition terminal and the ground terminal for each gauge? You have to have power there, for anything else to work. Make sure that the fuses don't have oxidation on them, I just had this problem = no ignition, gauges, blower or bilge pump. They all decided that they had enough salt air at the same time lol.
I did some poking around the dash under my boat which is 10 years older than yours and the wiring was in better shape than I thought. The wiring you showed, is why boats should have only tinned wiring even for fresh water use. I found a number of junction blocks under the dash, any one of which could have corroded terminals inside. I didn't want to chance pulling those apart, but I did open up the main harness connector that runs from the engine (big cylinder shaped connector with 8 or 9 terminals inside) to make sure the terminals were clean. They looked pretty good, so I coated the ends of each half of the rubber part of the main harness connector with grease and put it back together with a stainless hose clamp to keep it together. The ignition/gauge wiring comes from here as far as I can see in the OMC manual, but where it interfaces with the boat wiring gets confusing. F/Ws wiring schematics are very hard to read at least for the older boats.
http://www.fourwinns.com/upload/Documen ... orizon.pdfHere's yours maybe you can enlarge it and make some sense out of it. I looked at it for a while and it looks like the inputs for the gauges, come from the 8-pin main engine harness (on the engine), to another terminal block (under the dash) that appears to have 24 pins (maybe not all used though) and from there, the wires run from that junction block to the all in one gauge cluster. So there are 2 places at least where you could have trouble, one is at the main engine harness connector, the other is this large terminal block just before the wires hook up to the gauge unit. I'm thinking your problem has to be in either one of those or both....look especially at the wiring for the switched power from the ignition switch and the ground.