Tell me about Sonar/Radar

[Invader]

I'd like to know if sonar can pick up objects just beneath the surface of the water, perhaps a mere 10 or 20 feet below. I have a feeling that the noise created by the object might be enough to alert other detectors that a ship or submarine would have on board, but I'm not exactly sure. I know that there are ships/submersibles that utilize this "listening" technology, but whether or not it's used by all military ships or just a few of them is beyond me. I'd also like to know if small boats can avoid being picked up by radar, considering how low they are to the water as well. If a watercraft is small, and over the bend of the horizon many miles away, does radar lose it's ability to detect it then?