Im running a mechanical oil pressure gauge. I like it. I have run them before in cars. Mine uses a stainless steel braided line verse that clear plastic tube. 

its kind of a double edge sword!  mechanical or not.  mechanical is more accurate and is real time, however if that line to the gauge were to break or get a pin whole in it . you will have oil everywhere ! and if your not paying attention ten quarts of oil in your boat and a boat anchor for a motor!  replace the sending unit check the wires. call it good. 

Like said be fore double check your wiring but I could be possible that ur gauge mite be going out. I had a bud run a mechanical gauge and instead of the plastic tubing he ordered the copper tubing and it worked great.  I would prefer a electric gauge my self.

electrical here.  i dont want oil running all over the boat.  im not running a 1000hp race motor.  i mostly just want to know if i have pressure or not. 

That's Why I love this Dam site!! You guys always come through for people that ain't sure, Thanks for all feedback. I definitely don't have no race motor and just want to make sure I have oil pressure.  Guess I will buy a new gauge and try that before Trying a mechanical.

if you have a gauge right now, take the sender wire and ground it.  the gauge should do a full sweep.  if the gauge works then you might have a faulty sender (just because its new, doesnt mean its good) or it could be the wrong ohm rating for the gauge.  make sure the 12v and ground is good.  a boat isnt like a car where you can just run it to anything metal, having good grounds are VERY important.  also the sender works through a ground, so if you slap a buttload of teflon tape on it (it should be a NPT thread and should seal itself when tight) it could give you a faulty reading as well. when the ignition is on, the gauge should zero itself out, and obviously when running it should move around with the rpms.

When I pull the sending wire and ground it the needle does top out. when I turn the ignition to the on position the gauge doesnt zero out but rises half way. How would I tell what sending unit to buy for my gauge? I bought a brand new sending unit last week and was doing this, bought a new one yesterday making sure it was for a gauge and still doing the same dam thing. its frustrating because its the last thing to check of the list before starting interior.

what brand of gauge?

Just run an idiot light with a 10psi switch. When the oil pressure drops below 10psi the light will come on. If all you want to do is make sure you have pressure