I have been doing a fair bit of paddling with 2 different GPS's and the distance traveled between the 2 is quite a bit different, one is the Garmin the other a much more accurate GPS. Would love to know what most people use to time there paddling and if using a Garmin do you know it miss reads by quite a bit? in a 10km paddle it will be 300m difference which is alot!! Any other ideas to get accurate times out there?

How is the 5.0 thing in Sydney measured?