According to :
we asked the contractor - who is a very honest guy generally -
about it and he said something to the effect of "well, that's
something that happens sometimes out here and can't be avoided when
you're so close to the water" (the place sits about 25 yards from the
ocean). I was very skeptcal of his remark, but my folks basically
bought it mostly because they were happy with the majority of the work
and glad to be finally done with the project and able regain use of
their beloved cottage.

The contractor is mostly right. Coastal areas are hard on
lumber, and whether something like prefinished flooring cups or not
will vary somewhat between batches/suppliers/species, but is likely
to be present to some degree regardless. If the wood had been
dried to a lower equilibrium than the house, the unfinished side
will soak up moisture, causing it to cup away from that side.

The best way to minimize it is to let unplaned wood season and
equalize _in_ the area, and then machine it into floor planks _after_
it's dried out as much as it's going to in that local climate.
Which means a local manufacturer. You probably don't have one anymore.

When you import prefinished real hardware flooring from elsewhere
(which most stuff is these days), things like this will often
happen to one degree or another.

He might have been able to minimize it somewhat by priming the
backs of the planks. A lot of extra work for no real guarantee,
that I suspect nobody does.

Be patient. It's probably still equalizing. It may well get better
over the course of a year. But it might get worse. Watch the warrantee
expiration dates...

If it gets too objectionable after a year or two, get it resanded/finished.
That should fix it permanently - it'll have stopped moving, and
resanding will take out the cup.

A new public school built near us had a full size hardwood gym floor.
Unusual in these days (most new schools get much smaller gyms). Awesome
stuff from one of most reputable flooring manufacturers in the world -
the guys that do floors for professional basketball teams etc. Swedish
I think. To the level of having manufacturer's engineers fly over from
Europe to spec the floor, and then have inspectors onsite to supervise
installation by guys who had to meet their certification requirements.

These guys know what they're doing, and charge appropriately.

A year later, it had split. Very badly. 1/4" and more cracks everywhere.
Everyone appalled, especially the manufacturer. Fly whole new floor
over, fly over inspectors to try to figure out what went wrong,
and do again, no arguments at all, redo probably cost them close
to $200K. They still don't know for sure what went wrong. But
they stood behind it.

It happens even to the best.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.