It doesn't make sense because it isn't correct.N9149A wrote:...Interesting that there is no classes of aircraft in the Sport Pilot rating. So I could put my Cub on floats and let my medical lapse and fly it off water with no further instruction. But with a current medical I would be exercising at least my private privileges and need the sea plane rating. Doesn't make much sense does it.
A pilot with a Sport Pilot certificate needs instruction and an endorsement for every new aircraft type he or she flies. It's simpler if the new aircraft make/model is in the same category and class as one for which the Sport Pilot already has flight priveliges, but it still requires an instructor's endorsement. (That's in FAR 61.323.)
Because you don't have the seaplane rating on your Commercial certificate, you need to satisfy the requirements of FAR 61.321 before you could fly your J3 on floats under the Sport Pilot rules. That's the same as if a pilot with a Sport Pilot certificate was endorsed to fly a J3 on land, and then wanted to fly it as a seaplane (different category or class). You could do this regardless of whether your medical certification was an FAA medical certificate or a driver's license -- either way, you'd be restricted to the priveliges of a Sport Pilot because your pilot certificate doesn't authorize you to fly seaplanes. No big deal -- it just means you can't fly your seaplane at night or in furtherance of a business, or carry passengers for hire, even if you have a Class II medical.
FAR 61.321 says you need to get instruction in the knowledge areas specified in FAR 61.309 (nothing you don't already know), and the flight proficiency area of FAR 61.311, which includes preflight, seaplane base operations, takeoffs and landings, and emergency operations, among other things. After your instructor endorses your logbook that you meet the knowledge and proficiency requirements, you must complete a proficiency check with a different instructor. Then you fill out an FAA form (8710? I'm not sure), and get the endorsement (from the second instructor) that authorizes you to fly your J3 on floats.