Caution: If you think some of my other posts are boring or too long winded, better block me from your buddy list immediately!!! 
Interesting post Woody, good handle on the basics.
I would like to point out, in the interest of accuracy, that the K-band radar units we are discussing here determine target speed via doppler, not time delay measurements. Also, the target, in relation to the radar unit, is not showing a constant bearing unless it is driving directly towards (yikes, collision course!) or directly away from the unit. Other than those two situations, it is generating a bearing rate depending on its aspect to the radar unit.
For those not familiar with doppler, think of standing next to a railroad crossing as a train sounding its whistle approaches you. As the train approaches, the whistle has a higher pitch. The instant it passes you by, the pitch is the original frequency. As it drives away from you, the pitch shifts to a lower frequency. That's doppler, in a nutshell.
The same principle applies to the handheld radar units under discussion. A carefully calibrated narrowband radio frequency is transmitted from the antenna arperture in a narrow conelike beam. At maximum range, that beam, only a few millimeters in diameter at the gun, has grown in size to several hundred feet. Any target within that cone generating relative motion in relation to the radar gun will return some level of signal. The greater the relative motion, the greater the speed displayed on the handheld unit. Unless the relative motion is dead on or dead away from the transmitter, any angle will show a speed lesser than that actually being conducted by the target. The greater the angle, the less the relative motion, hence less doppler shift, hence a displayed speed less than actual speed. The stationary radar does not care if it is down or up doppler, it just measures the difference of received frequency from transmitted frequency, and then generates a correlating speed number.
The other problem is the unit, utilizing what is known as the "capture effect", will display (in a multiple target situation) the unit producing the strongest doppler return signal. This is not the fastest target, or the closest target, or the biggest target. It is the target that due to its surface characteristics is returning the largest amount of signal at a given moment. In a multiple target situation, this can shift from target to target almost instantaneously. That is why these units will blank the dispaly when receiving multiple return signals, to minimize operator error.
Problems encountered by terrestrial radars are greatly amplified in waterborne situations due to platform instability, enhanced ranges and ghosting by signal reflection off the water, signal ducting caused by low level temperature inversions over the water, etc.
However, a trained and experienced operator should be able to compensate for these issues and operate a radar unit properly, albeit under limited and carefully controlled conditions.
Also, you are correct that there is no significant electronics difference between the terrestrial and marinized units. The only electronic difference is the ability to display speed in tenths of the measured unit (either knots or MPH), the ability to choose between knots or MPH display, the color white and a heavier rubber grip. Oh yes, I almost forgot the most important difference. The Marine version comes in a case that floats, vice the standard aluminum case that does not!
Finally, it will be a long time if ever that you see a marine "fire control" type radar unit used for speed analysis on an inland body of water. First, it would take three people to operate the platform correctly (radar operator, spotter and boat operator). And unless you were lucky enough to score someone with a military background, the cost to procure and make someone proficient in these devices would be cost prohibitive for an agency that depends heavily on seasonal help.
By the way, (as some of you have already guessed) I have been operating portable speed measuring devices since 1975. I also have six years active duty submarine service as the Sonar LPO on 640 class ballistic missile submarines with a specialty in narrowband doppler analysis, and ten years reserve duty with the Naval Inshore Undersea Warfare units as a watch officer, where I spent countless hours utilizing various Raytheon surface search radar units.
Yes, I know I am just an amateur at this stuff, but I always like to throw in my $.02 when the discussions get technical.
As always, feel free to e-mail me if you would like any amplifying information.
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