Final answer:
High tide appears to be the regular occurrence at midday, as evidenced by references to the run of the flood-tide and the daily tidal changes observed in coastal regions.
Step-by-step explanation:
Based on the information provided, the event that seems to be happening regularly at midday is B) High tide.
References to the run of the flood-tide and the mention of Boats in the Bay of Fundy at high and low tides suggest that the tides are a significant daily occurrence. The tidal phenomenon is caused by the gravitational pull of the moon, and at most coastal locations, high tides occur twice a day. Mention of the rotation of Earth carrying an observer into regions of deeper and shallower water supports this conclusion, explaining why an observer would experience two high tides and two low tides in a single day.