Closed raw water intake -- no fuel pressure

[SeaHarlequin]

I recently installed a fuel pressure gauge to monitor my electric fuel pump and it appears to be working fine -- the PSI fluctuated between 2 to 4.5 PSI while idling for about 20 minutes.

I shut off the raw water intake and opened a bypass intake which was immersed in a bucket of fresh water to flush out the salt water. I noticed the fuel pressure gauge stopped registering any pressure even though the engine was still running. I thought maybe it got stuck or the fuel pump turned off (I had the engine running for less than a minute so presumably there was sufficient gas in the carb bowl to continue operation.)

Out of curiosity, I repeated the steps -- opened up the raw intake, observed PSI readout within expected parameters, shut down the raw intake and started fresh water intake and observed the PSI readout drop to 0.

I'm not yet understanding the interplay between intake of two sources of water and the fuel pressure. Is this expected behavior, and if so -- why?

I've not had a fuel pressure gauge on this boat before so maybe jumping between pressure and no pressure based on engine needs is normal and I just happened to time things twice so that it looked like there was interplay? (I'm operating under the assumption that an electric fuel pump would provide consistent pressure and was surprised to see the needle jump between 2 and 4.5 actively, although I didn't see it drop to 0 during those times.)

Thanks and apologies for what is probably a stupid question/mis-timed set of observations but I'm trying to get a better understanding of how the systems work together.