I’m not sure about your motor specifically but my 2002 had a brass plug in a threaded hole below the starboard cylinder head where a barbed plug can be installed to plumb in a water pressure gauge. If your motor doesn’t have said plug, you can use the water hoses off the thermostats and plumb in a t-fitting in the hose with a barb fitting to run the pressure tube to a gauge. Mercury and others sell water pressure gauge kits that come with all the parts you need.