Gotcha, Ok. It does not need to be documented in order to charter it. It's an oupv "6 pack" and only needs to adhere to requirements for those. I assume you have your USCG 6 pack or masters license??
The state requires state registration even if it is documented (they need to get their cut of taxes...) It's a grey area, some people dodge taxes by documenting only. They look for it.
I would imagine, a small boat like that, if it didn't have state reg numbers, you'd run the risk of getting hassled by law enforcement because the only other reason for it to not have numbers is if it's a tender to a larger vessel. Even if it was a tender, you can technically only operate it as a tender, for part of the year, and they do get hassled as well.
Bottom line: State reg the boat. Get your 6 pack or Master's license. The boat does not meet the 5 ton requirement to document. The boat is too small. Generally, a boat needs to be around 30ft +- to actually meet the requirement. And, even if you padded the numbers a bit to get to the 5 ton requirement, you'd still be dodging state taxes. Also, having state reg numbers keeps you from getting pulled over regularly.