You may have to put another mechanical oil pressure gauge In place of the sending unit to see if the readings are true.
Edit: For got to mention last night should this engine be an MPI 'fuel injected') 'See if someone like a dealer or friend, etc. can run the engine with/for you while connected to a marine ECM scan tool to see if the oil psi reads the same as the dash gauge because there are already 2 different oil sending units on the engine that read oil psi...One is digital while the other is analog...The digital is read by the ECM while the analog is read by the helm gauge.
Make sure the oil level is full and know what engine oil weight and brand was used.
If you can verify that everything above mentioned is within specifications including 2 different oil psi reading from either a scan tool or another gauge connected directly at the engine and you still see a low psi reading on all the gauge readings, then as two or three quick tests you could try doing would be first try a new oil filter hoping for a higher reading and if not then either try a higher weight oil like 20/50 to see if it goes up any and/or with the oil hot already, try temporarily overfilling the oil level with about another 2 more quarts of oil to see if the psi goes up to a more normal reading.
If the oil psi goes up with adding more oil (or even adding a higher weight seemed to work too for this test too) then it has been my past experience with engine models around your model year that the oil pickup tube "Screen" had fallen off the tube laying on the bottom of the oil pan and by adding oil temporarily raised the oil level for the tube to help suck it up better.
https://www.ebay.com/itm/Screen-Oil-...4AAMXQVERSvapM
https://www.mercruiserparts.com/bam/...31849/8093/140
