Typical boat speedometers are water/air pressure driven devices. They usually starts indicating speed when the boat reach the 20-25 mph range. The way these speedometers work is a pick-up tube (pitot tube) is placed in the water with an opening facing forward. This tube is connected to the speedometer by a small rubber or plastic hose. As you drive along, water is forced into the tube opening which squeezes or compresses the air in the hose. This, in turn, moves a piston to rotate the speedometer needle.

       With that simple description, you can probably come up with a couple of ways for things to fail. First, the pick-up tube opening can get plugged. This opening can be on the leading edge of the big motor lower unit, or it can be a hard plastic device, usually white nylon, mounted to the transom. Either one of these openings can be cleaned with a small piece of wire. Be careful not to change the size or shape of the opening when cleaning it or you'll change the accuracy of the speedometer, which probably isn't very accurate to begin with. If while cleaning, you manage to lose some of the stuff down inside the opening, disconnect the hose at the speedometer and blow into it.

       If the pick-up opening appears to be okay, maybe the hose has a break in it. To check that, disconnect the hose as close to the pickup tube as possible and blow into it. You should be blowing into a dead-end. If the air just keeps on going and going, find where the hose is broken or disconnected. I guess another way to put this blowing stuff would be to say, under normal conditions, blowing from the speedometer end should be easy, while blowing from the pick-up end should be difficult.