Correct, the basic umbrella policy is a multi peril liability policy that will cover you and your boat, cars, bikes, etc etc, in excess of any other policies. It also provides for defense of lawsuits brought against you.
But you can add UM/UIM coverage to it. Basic policy typically $1 million in excess liability coverage. It usually requires you must maintain adequate underlying limits on your cars, mrtorcycles etc. That is usually 250/500 liability and UM/UIM.
Your agent should be well versed in all this information. However, my experience around here is the agents "staff" gets involved in a lot of these issues and many times are clueless.
I found this link on a quick look. Looks like it is from Wisconsin but has some good info. May not all apply to every company but it gives a general overview of coverage. Never heard of the company sponsoring the site.
http://www.weabenefits.com/uploadedF..._Insurance.pdf