Are these tarmacked areas where the boats are pulled up onto, or areas of water where there tied up? If there areas of tarmack maybe just a different amenity= tag would be suitable, if it’s a mooring area on water, then waterway maybe. I know little on this though, so just saying the key’s that would seem ok assuming it’s one of those 2.
In general I think these are areas of grassland. Dingys are stored throughout the year with a waterproof over the crew area. Whereas boatyard is probably the most appropriate, these areas are clearly not waterway, nor are they generally amenity: these are usually private storage areas within the boundaries of the sailing club. Large marinas will also have this type of facility (often with a crane), and this is used for storing keel boats (either over winter, or if raced seriously, between races). In this latter case the boatyard is the most appropriate tag.
My primary concern was to remove erroneous amenity=parking information from the database, but its always nice to be constructive in replacing such information.
waterway=boatyard is the only waterway tag that isn’t water, a way, or a feature on a water-way. I think that’s why I see it as not being a particularly good tag. It’s just linked to areas which operate around water. A building at such a site, which is an office, is linked in the same way, but wouldn’t be waterway=office just because of this link.