Take a look at the back of the gauge and see if there's a vinyl tube connected to it.
If so, you've got a blocked pitot tube. On the leading edge of the lower unit, there is a tiny hole (you can run a thumbnail along and it should snag at the hole.) As you move forward, water gets forced into the hole and it pressurizes the air, and that pressure is converted to a speed at the gauge. Unfortunately sand, kelp, and bits of shell all love to try and hide in the hole. Clear it out with a paper clip, small drill bit, or similar, and you should be good.
If it's an electronic sensor, well, I don't know. Not sure how the networked Yam's determine speed.