You need something to read what is availale to you, whethere that be a digital speedo signal or an analogue temperature reading. There is interface cards available that will connect to you computer via usb or serial and will provide such readings.
You then need some way of calibrating your computer to say, 1v = 70degrees, etc.
Something like this provides 5 digital and 2 analogue inputs. Depending on the age of your boat and gauges etc, you may need more or less of each input.