The HIN can be of value. I've heard plenty of people say "what year and model is my boat" and sometimes, owners are listing it wrong. Having the HIN can confirm just what it is, and perhaps confirm problems with a lien, title, etc. as well. Publishing the HIN can also be bad, because it means someone else now has a valid HIN and can use it to fraudulently "clean" the registration on another boat of that model. (Yes, HINs are altered and forged that way.)
On the other hand, that service sounds like the CARFAX service for cars. In theory Carfax is great, it lets you find out if the car you are looking at has been in accidents, etc. In practice? Carfax has a corrupt database and takes no remedial actions about errors reported to them. I'm the original owner of my car, I know exactly where the title is and has been. But according to Carfax, a second or duplicate title was issued on the car, without my knowledge. Carfax is the only place that "knows" about this other title, but they refuse to respond to questions or complaints about their records.
With policies like that...who can believe anything they say?
If any service doesn't provide good customer service, forget 'em.