This story is a sequel to Ditzy Don't
When Spike and his friends go on a trip to Daypony Beach, he's rescued from the sea by a strange filly. Despite having reservations about this mute pony, Spike and his friends welcome her into their group. However, the restless ocean--as well as the innkeepers warnings about the mysterious "ponies of the sea"--has everypony nervous, and when a nearby clan of dragons becomes involved, things go from bad to worse.
Stories in the Ponies of Olympus continuity (chronological):
Ditzy Don't
Rolling In Beaches
Head of a Dog; Tail of a Lion
Somebody To Love
Atlas Strongest Tournament