Are you getting any other values when you spin the knob or does your meter just read infinite? Are you sure your meter is good? What type of meter are you using- Digital (DDM) or analog?
How are you measuring the resistance? Do you have a three wire or two wire sending unit? Not to insult and for the benefit of anyone else who comes along looking for answers, an infinite resistance reading suggests an open circuit, ie a broken wire, meaning the meter doesn't know what to read because there is no signal coming back to complete the circuit. A broken wire is the most likely scenario followed by a bad sending unit, which is basically just a potentiometer or pot.
Are you reading resistance from the dash or is the sending unit on the bench? Did you insert your multi meter into the circuit as close to the sending unit as possible so you can rule out a break in the wire between the dash and the outdrive?
Depending on where you are in the troubleshooting, you can try splicing the wires which run from the dash to the outdrive together at one end and then using your DMM to check for continuity at the other end across the leads- disconnect the ends at the gauge first however. If no continuity then you have a break which is great and easy to fix.
