Dacha or more

Море - sea

Дача - dacha, holiday house in a country-side

These people are, obviously, at the sea-side, not at the dacha (not at the Russian one).

"На море вместо дачи?" сan be translated as "To the sea instead of dacha?"

After "вместо" we can see "дачи", not "дача" - we need Genetive case here.

For most people spending holiday at the sea coast appear a better alternative to the dacha's dullness.

The answer is "Возможно!", which means "Possible!"