So the new to me boat that I purchased this spring is in need of a complete dash upgrade. I have done this before on my 27' Fountain but this situation is completely different. This is why I need some guidance.

My boat is a 1997 and it has twin 496 Mag Ho motors from 2003 in it. Who ever rigged the new motors in the boat did not hook up or the existing gauges except for a few must not of been compatible..?

What gauges can I or should I have besides the obvious Tach, Oil Pressure, Volts, Fuel and Speedo..?

oil temp, water temp, water pressure..

I was told by the previous owner that water pressure could not be hooked up to those motors..?

check your engines for sensors, you may have them but not connected or may need to buy them along with gauges. Water pressure can be picked up at a drain port on the block.

My 06 38 EC has mechanical water pressure gauges. Basically, they installed a brass tee where the mercruiser pressure guage is installed in the oil cooler at the rear of the engine and ran a 3/8" heavy duty water hose up to the dash for each water pressure gauge. When I installed the smart craft sensor I can check water pressure both electronically and mechanically and they are pretty darn close. The mechanical gauges appear to be less acurrate at lower pressures (2-5 psi) and real close at 20-25psi. In my opinion no better gauge than mechanical and in this case its only water running up to the dash. Obviously, you would run mechanical fuel pressure gauges at the dash. Don't forget to post pics as you proceed.

Should one decide that they must have mechanical gauges for fuel/oil/etc, the race car shops sell isolators that allow you to use an inert liquid (antifreeze I think) in the instrument line.

Like below-
http://www.jegs.com/i/Mallory/650/29806/10002/-1