There are several ways to measure the  speed of your Rc boat or anything for that matter. The first and cheapest would be to measure the time from one known point to another known point. With Rc boating that way can be a bit of a challenge. First of you would have to set up markers, which is a pain in itself. Then use a stop watch to time between the two points. There will be a high margin of error doing it this way and slight performance gains will be hard to measure this way. You can also use radar but that takes two people, one to run the boat and another to use the gun.

          We use a Garmin Etrex GPS. This unit is very small and keeps in memory the top speed achieved. Because its placed inside the hull, only the person driving the boat is needed to fine tune things. The only downfall is the cost, approximately one hundred dollars. These are also very accurate, Garmin states within 1/10 of one knot variance. One plus is that this is a full fledged GPS unit that has many other uses like hiking and hunting. More information can be found here Garmin.com

Measures 4.4" x 2.0" x 1.2"