The first thing that needs to be found out is IF the pressure is actually dropping. Oil pressure gauges, and the sending units that feed the gauges a signal do go bad. On top of that, if there is corrosion in the wiring between the sending unit and gauge that can cause erroneous readings. What we would do in the shop is remove the sending unit from the engine and screw in it's place a mechanical oil pressure gauge and take the engines oil pressure manually. If the mechanical gauge gives a higher reading then we start looking at a bad sending unit or gauge.
If oil pressure is low on the mechanical gauge, step 2 is to change the engines oil and filter, say a little prayer, and see if pressure comes back up or not. A clogged oil filter can give a low reading on the gauge.
If pressure is still low, then your down to 3 possabilities. There is either 1. Sludge in the engine, 2 a bad oil pump, 3 bad crankshaft bearings causing a drop in pressure.
In any event, if the pressure does end up being low the engine will need to come out, oil pan removed, and the pump and bearings need to be inspected to see what is faulty, or to see if there is sludge in there.
Is the engine safe to run? The oil pressure is below spec. Spec is 30 at idle and 60 at 2000 rpms. You can probably get away with running it at a low rpm if you need to move the boat to a dock/launch, but I would not run the engine hard.