Learn Swedish: Flaskposten guppade på vågen. - The message in a bottle bobbed on the wave.

Flaskposten is a compound of en flaska (a bottle) and post (mail/post). In Swedish, when a noun ending in -a becomes the first part of a compound word, it usually drops that final -a. The -en at the end is the definite article, making it "the message in a bottle".

Does the verb att guppa only apply to things floating in water?

Not exclusively, though that is its most common use. Att guppa means to bob, bump, or rock gently up and down. While it perfectly describes boats, ducks, or bottles on the water, you can also use it for things on land, like a car bouncing along a rough, uneven road (bilen guppade fram).

Why do we use the preposition på instead of i for the wave?

Because the bottle is floating on the surface of the water, Swedish uses på (on), which mirrors the English expression "on the wave". If the object had sunk and was submerged beneath the surface, you would use i (in) instead.

Vågen comes from the root word en våg (a wave). Since it ends in a consonant, how is the plural formed?