Technologically, there is no primary difference between marine and land-based systems. However, depending on the frequencies required and the focus of the applied system there can be significant differences in size, physical positioning, target resolution and powered-range. In the case of marine systems the goal is to see what is front and around the vessel out-to-the-horizon, so typically RADAR emitters are usually positioned at the top of a mast, or other vertical structure, in order to maximize the line of pulse.
Once the system is employed it will then be able to "see" other vessels, land masses or other potential obstructions out to the limit of the emitter's range. However, the size and power-generation capability of the particular vessel is critical, since marine operations make their own primary and ancillary power. Therefore, if a vessel is a large one the power plant will consequently provide better RADAR ranging and target resolution, than a system that is limited by the available power of a smaller private boat, for example.
RADAR operates on the principals of what is referred to as a "Doppler Shift." If a system pulses a signal at a particular frequency and encounters an obstruction the original signal then "bounces back" to the receiver and measures the time the response took in terms of milliseconds, while also measuring the density, amplitude and frequency of the return signal. This electronic response is then converted into a "plot-paint" represented on a video screen. This subsequently creates a two-dimension visual representation of the vessel's particular position in relation to the target of interest.
As mentioned earlier, power consumption is critical to marine operations. The range of a RADAR will be limited by the strength of the signal, so in the case of small vessel co-generation the range of the system will be consequently constrained.
Second, the size of the array or rotating emitter will be limited in terms of the resolution of the system. In this case, large system can "see" down the the smallest obstruction on the sea, where smaller arrays may or may not lose this capability depending on the quality of the particular equipment.
On the pro-side of the equation, however, today's marine RADAR electronics are very cost-efficient and highly-tuned systems, and even small vessels can take significant advantage of the continual technological development.
Because of the need for continual marine miniaturization, many of the better vessel systems now integrate commercial Global Positioning Systems (GPS) in concert with RADAR technology. This capability allows a vessel to locate itself on the earth down to the meter of position. The ability to not only know exactly where the vessel is at any given point in time, plus the ability to "see" the specifics of the surrounding sea environment, can be a life-saver to a mariner who might be in trouble.