Is the purchase of a newly built house boat, which will be used primarily as a rental, HMDA reportable?

No, they are excluded from the definition of a dwelling.

"3. Exclusions. Recreational vehicles, including boats, campers, travel trailers, and park model recreational vehicles, are not considered dwellings for purposes of § 1003.2(f), regardless of whether they are used as residences. Houseboats, floating homes, and mobile homes constructed before June 15, 1976, are also excluded, regardless of whether they are used as residences. Also excluded are transitory residences such as hotels, hospitals, college dormitories, and recreational vehicle parks, and structures originally designed as dwellings but used exclusively for commercial purposes, such as homes converted to daycare facilities or professional offices."

Adam is right, but I want to clarify that this began in Jan 2018. Prior to that, a house boat was a dwelling for HMDA, if it wasn’t recreational.