By far the best alternative is to get a GPS unit and use the Speed Over Ground. It is not affected by current or waves and is very precise. In the last couple of years the government has allowed the signals from satellites to travel to the GPS units with very little scrambling, so even at slow speeds you will get a good indication of speed. You can either use a Sonar/GPS combo unit – or even a handheld GPS will do the trick. The other advantage of a GPS is that you will have plot trails so you can reproduce productive passes.