WTF

Yup. As someone who's worked a lot on ships all over the world, I can tell you that waves can be deceiving. It can look like everything is normal, but suddenly the ships roll and pitch will perfectly time a slightly larger swell which then gets to wash in over deck.

It is worth noting that the ships themselves have no problem handling this. They're designed so that any water that washes over the deck gets drained out and doesn't go into the interior. Whoever is on those decks is another story though. Just because your feet are dry, don't assume they will remain so.