It's not bad for a few minutes, but I seemed to reach a point where there was no longer any point in building defenses.
That was not just because they'd be washed away and I couldn't build enough, but because my entire back row of churches and one statue, and two specific spots in the row in front (fourth and sixth column), seemed to be immune to the tide anyway.
I feel like this should have some end goal, where you either aim to achieve a certain number of sand dollars, or get inevitably wiped out after a while and play for a high score.