Fridge Brilliance: Why do the Kirbys suddenly need air underwater? Due to being split up and weakened, they simply lost the power to breath underwater, just as how they lost the power to copy.
That, and they're too small to wear his snorkel mask.
Except that a snorkel's useless if its tube doesn't extend above the water's surface.
Fridge Horror: In this game, Kirby dies. A lot. And being split into ten Kirbys, he also gets to watch himself die repeatedly, sometimes in quite gruesome ways. Imagine watching yourself get crushed, incinerated, mangled by machinery, eaten by monsters...the list goes on, and it just keeps happening, again and again. This could be the darkest game in the entire series. This doubles as Tear Jerker when you're brought to the game over screen if your whole Kirby platoon gets wiped out by a boss.
Fridge Logic: Okay, so the Kirbys need air to stay underwater for extended periods of time. Understandable enough. But how is it they don't have the same air-problem in space? It is a problem harder to solve but still. Daroach lampshades it in one of his general tips in Volcano Valley.
For the same reason Super Mario Galaxy's ability to travel in space unaided doesn't extend to underwater either?