Why do people say "over-" and "underwhelmed" but never just "whelmed"?

If a boat is whelmed it means that waves are coming right up to the gunwales, the tiptop of the sides of the boat, and some water is sometimes coming into the boat. This is something you can cope with but isn't pleasant. There seems to be little use for this word in a non-jargon or metaphorical sense.

When a boat is overwhelmed, water is just pouring over the sides and into the boat. This is almost certainly going to lead to sinking, capsizing and other horrible things. The word overwhelmed became hugely popular as a metaphor for anything you can't cope with that is sinking you.

Underwhelmed is a backformation and works only metaphorically. Nobody says "the weather is lovely, the sea is calm, the boat is underwhelmed." It started as a jokey comment, much like saying an actress can display the whole range of emotions from A to B, but these days is used with sincerity and no sense of wordplay by people who just see it as a synonym for "disappointed" or "not excited."

Well, it's mostly used in a satirical sense now. Like in the cartoon Young Justice, they use it just for humor. Where Robin likes to take pre-fixes off of words that are used often in the English language like Overwhelmed. I don't see it being used in common language.

I'm surprised no one brought it up from Moby Dick, which I'm reading for the first time.

“I leave a white and turbid wake; pale waters, paler cheeks, where'er I sail. The envious billows sidelong swell to whelm my track; let them; but first I pass. ”