Speedometers on boats are useless. Not sure why they even still put them on one. All it takes is a little dirt or sand in the pickup tube and they will no longer be accurate.
In my opinion don't worry about it. Even if you get it to be a little more accurate it wont last very long, as the first piece of leaf, grit, or sand that gets back in the tube you will be off again.
I would suggest a sonar, with built in GPS if you want a more accurate speed.