Have you used one of these plug holes to put the connector into the pump housing? (Blue plastic plugs; the pump has them too...)
What about this specific plug location, (the blue one in the picture) on the PT side water drain vent?
The water will have passed through the STB side, the back, the oil cooler and the PT side before coming to this point. I reckon this would enable me to monitor a greater route by attaching it here. What do you think? The pressure should drop more closer to the end of the loop if there's a leak somewhere, right?

I have an early 496. The factory sea water pressure sensor in located on the sea water pump. I installed a tee in the pump to add the water pressure gauge.

I went to a local hose shop and had them make me up quality hose the length I needed with my requested ends. Yes the plastic tubing will work but I don't trust it for long term. Do it right, do it once imo.

Could pull the water pressure sending unit out and put a tee in there.

OK, thanks! That sounds reassuring!

OK, I see. I'll have a second look at it. Perhaps a local hydraulics shop could fix me a tee as well as a heavy duty hose! I'll try that! Good suggestion!

Thanks for clarifying!

To everyone contributing here: A BIG THANK YOU!
Now I know how to fix it thanks to you guys!

Those blue plastic drain plugs have a funny thread and oring setup. I don't think you want to mess with trying to adapt that hole. Like was mentioned, you could T off your factory sending unit on the water pump, or if you have a 1/8" pipe thread drain on one of your coolers.

OK, Thanks! None of the coolers seem to have drain plugs, but maybe I can cut the hose somewhere and clamp on a t-fitting, or I'll go with the hole for the pressure sender. (The advantage of having the hose for the gauge as late as possible in the system would be that if something gets clogged somewhere and the pressure falls in som parts it would still be discovered, which I'm afraid it wouldn't if the pressure builds up right after the pump...)

Will I have to drain the gauge tubing for winter to keep it from freezing and cracking? (It is now way below freezing temperatures in the boat...)

I'm not sure about the freezing question. Hopefully someone who lives in a cold climate can answer that. Sorry