Your engine has the oil pressure sender mounted on the front top of the engine, just a little starboard of centerline.
This is NOT a 4.3L engine that some of the people giving you advice seem to think it is. This engine has variable valve timing and high pressure direct fuel injection.
Oil is supplied by a variable displacement two-stage vane-type oil pump assembly. An oil control solenoid valve, controlled by the engine control module (ECM), mounted to the oil pump provides two stage functionality. The pressure relief is in the pump and a bypass valve is in the oil filer housing. There is also another electric solenoid at the front of the cam shaft.
This engine really needs to be connected to a scan tool operated by someone with an understanding of how it works. There are multiple fault codes for the oil pressure due to having two electric solenoids in the system.
Yes the speedo problem could be related, it depends on what gauge package your boat has installed. I worked on a Volvo powered boat that the trim gauge didn't work, trim gauge was good, trim sender was good, ECM was good, wiring was good. Problem was a bad tachometer. Tach worked good the only part of it that was bad was the section that interpreted the CAN bus info and sent it to the trim gauge. On your engine the oil pressure data comes from the ECM, how it gets the the oil pressure gauge depends on what gauges you have installed.
If you change the oil, measure how much oil comes out. There's a possibility too much oil is in the crankcase. Maybe someone put the wrong dipstick in it?