Thanks everyone for your posts. It turns out that the solution was much weirder and more serious than a sensor issue. TL;DR: check your fuel lines immediately if your pressure doesn't read correctly.
Background: I changed out all the fuel lines a couple of years ago to make them compatible with ethanol-containing fuel. I did it with the mechanic who worked with the club I was a member of at the time. He recommended only doing the supply lines for reasons that I didn't think to ask at the time.
Fast forward to these past few days. As I mentioned, the fuel pressure sensor was (and is) fine. So I went up into the engine nacelle and disconnected the two lines that run from the 4-way splitter down into the cabin, one for the return line to the header tank and one for the fuel pressure sensor. What I found surprised me.
Both lines were of the clear flexible type, except both were rock-hard. I had to cut them with an x-acto blade to get them off. Now I had to figure out which was which. Blowing into one caused a bit of fuel to come up from a vent line, that was clearly the header tank. Blowing into the other made a hissing sound, which sounded very close. Odd.
To check if this was the correct line, I disconnected the sensor again up front and put a bottle over it to catch any fuel. Blowing again, I heard the sound but no fuel came out. Odd.
Time to take the cabin apart. Behind the seats are two panels and a trim piece. All removed and ... holy moly. The hull (it's a boat when it's on water, remember) had fuel sloshing around in it! But how?
Examining it more closely I could see that a normal fuel line came out of the 4-way junction but a very slim one led to the sensor. Somewhere there was a mating between these two... and it turned out to be *inside* the heat shield sheath running down from the nacelle. Completely hidden. Even more terrible, it was just this hard plastic line with the smaller line *zip-tied* to secure it. When I was blowing on the line, it was the leakage that I was hearing. I pulled on the line and it came apart with no effort.
Clearly this was a bad design from the factory or maybe a worker cutting a corner. Either way it had to go.
I (slowly, painfully and frustratingly) ran two new fuel lines down to replace the hard plastic ones. One now goes to the header tank and the other one now goes into the cabin where the smaller line is secured with a hose clamp. It's also now able to be inspected for leakage should it ever occur again. All of this should have been done when the other lines were replaced of course.
With everything secured and the fuel spill cleaned up, I started the electric pump and, success, fuel pressure displayed on the EMS. I also ran the engine and the additional pressure from the mechanical pump registered just fine.
So that's it. I was certain it was a sensor problem and I was wrong. This is a design issue which is possibly just with my SeaMax (S/N 100) but if you've got one, it might be worth checking yours just to be sure.
