Never heard of the laser guage... If you really want something to accurately measure fuel consumption I would suggest a flow scan. It gets installed on the fuel line leading to your engine, measures the fuel and calculates your actual consumption.
In addition to knowing how much fuel you've used it will tell you have much fuel you have left and you get some other nice to know information that you wont be able to get from something that tells you how much fuel is left in your tank. Things like gallons burned per hour which will help you determine the best RPM for your boats fuel consumption and will give you range until empty.
http://www.floscan.com/html/blue/index.php
If you have a newer GPS some companies like Garmin also sell an add on module which connects to your fuel line and does that same thing and interfaces to your GPS.
Only thing you need to be aware of and every boat is different is your useable fuel... In other words how many gallons you may have left in your tank that can't be used because of how the fuel pick up is plumbed.
