For me its quite common to see a sea being partly frozen. On Spitsbergen I see this every summer. However I had never seen such a thing in the Netherlands. Even though we had quite a cold period for a while here, I was really surprised to see the Wadden Sea being partly frozen. Not heavy pack ice of course, but still large ice floes covering large parts.
On the first day of the new year most of the sea ice had gone already, but the salt-marsh was still largely frozen, giving a it bit a polar desert look.