@Lex79
From what I have read, which is still a very limited selection, there may be 1 main beach event per every 4 or 5 stories where the focus is the actual beach/ocean trip. Usually if the beach is somehow involved, it is just used as a location where most of the story happens elsewhere and/or focuses on a particular event or setting that is just "beach themed".
Mainly it is because those kind of fanservice dumps are a pretty rare thing for these types of stories. They don't need to entice the reader with jailbait in order to make a sale because their plotline is usually capable of carrying the show on its own, at least more so than most of the shounen generic trash that follows the same exact template plotpoints that every other shounen uses.