Let’s tackle this question in two parts - first, for floating debris. We get this question quite a lot, especially because here in the Pacific Northwest there are so many floating logs and dead heads. The answer is very unlikely, especially since radar is not sonar. Radar and sonar are similar in concept but their difference lies in the type of medium they operate: radar in air and sonar in water.
Now on to boats. You might see kayakers but it all depends on their radar signature. With boats it varies too and that is why it’s always advisable for mostly fibreglass-make (i.e. plastic) boats to have a radar deflector which will enhance their radar signature. In closing you’ll see most boats on radar (better on HD) but small runabouts will be harder to capture.