There are companies out there do "bolt on" NMEA2K sensors, and others do adaptors, so you buy the NMEA adaptor & connect in off the shelf analougue sensors so you could, for example fit water pressure, temp etc, and bus them all up down a 2K cable for the trip to the console (can also then add a sounder and not need to run another cable!). The down side is you would need to fit them yourself, although if it's a Merc based Mariner there may be a place to bolt a thermal sensor on the head. Water pressure tapping might be a bit more interesting to fit... From memory there was a pulse - 2K converter for RPM on someone's list.
A few to peruse, just google "NMEA2000 sensors" for more.
I half looked at it, but decided to put on the back burner 'till I build the console instrument pod extension so I have somewhere to put the plotter!