Most local authorities do not know the regulations so you will get a lot of conflicting information. An aircraft is not a boat so no boat license is required. Boat regulations regarding flotation devices also do not apply; common sense does. In effect that means use an acceptable flotation system, it does not need to be Coast Guard approved.
First requirement is to determine if you are on "navigational water", part of a water system such as the Inland Canal, Mississippi River, Great lakes or any of many smaller water areas that come under Federal control of the waterway. These areas are the easiest to determine the legal requirements. In these areas Federal jurisdiction generally begins 200 feet (if I remember right) from shore.
Inland lakes may have local requirements. Research State, County and community requirements and regulations. Do not expect a 100% straight answer if you contact the local sheriff. As indicated above they are not necessarily familiar with the regulations but it does not hurt to talk with them. Someone will surely report this strange craft and they will come to talk with you. If you will respond professionally and know your legal standing you will have won the battle.
The Seaplane Pilots Association is a good place to start. They have done much of the research and will add your input for other members. Some states have open waterways for aircraft unless a specific area is specified closed, others are closed unless specified open. However, keep in mind that an ultralight is not an aircraft with regard to some regulations. Good luck and happy flying.