I've been looking quite heavily into this (For an RV, though, not a boat) and this is what I've come up with (NB: This is purely a shopping list at this point - no testing).
In my case all of my sensors work with variable resistance (So, for example, with my temperature gauge the resistance goes down as the temperature rises).
This means that we have to measure the voltage drop over the sensor. The easiest way to do this is to have a static, regulated input voltage.
So far, the best bet seems to be here:
http://www.phidgets.com/
The 8/8/8 Analog input device and the voltage sensor.
Here is the diagram of how I'm (planning) on doing it.
With the blue square beign the standard sensor, the red square being the Phidget Volt Sensor and the blank square beign a resistor. The power supply would have to be a well regulated source to give an accurate calculation.
It would then be a case of either contacting the sensor people and seeing if they can provide some calibration info, or actually sitting down with your current dial/display and working out what voltage corrosponds to each reading.
Then it's just a case of plotting a graph and doing some maths in VisualBasic or the programming language of your choice.
Sadly, I'm finding precious little resources for this kind of thing - so it's been quite a challenge for me just to get this far as I'm in no way an electronics expert.
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