The radar on most large ships is usually located on a mast, which is not located on the bow. The radar beam sweeps out to detect other large vessels or land.
The beam is not directed down to the water until it is quite a ways forward. The beam can easily reflect off another superstructure sticking up tens of feet above the water line. But remember, even with a mast 18 ft high with a passive reflector, the beam has to reach that low to pick you up.
If you have the chance, walk toward a large ship at dock. If you can see the radar, the ship may be able to detect you. For many ships, that distance is quite a ways in front.
And the crew are located lower and it is even harder for them to see over the bow for any small craft.