I don't know of any laws that require insurance, but I am very sure that common sense does.
With all due respect to OP and to the very knowledgable experienced people on this site, your question should be directed to your state, your broker, your lawyer or your common sense.
It all depends on where your boat is registered. In Florida, currently you do not need proof of insurance to register a boat. If you are operating a boat that requires a registration you really should cough up the $150-200 a year to ensure you're covered if the worst happens. God forbid you get really hurt out on the water where your auto and or home owner's policy doesn't cover your medical expenses. Or worst, you hurt someone else through your own negligence and lose everything you own to a lawsuit. I'm of the belief if it moves you from point A to B in public... have it insured.