Is "Underwater" a Noun?

In Montessori, words are color coded as to their parts of speech. See below

How Montessori Teaches Grammar

The Chalk Game

We have a lot of Science materials from Waseca Biomes, and some of them include Grammar work as well.

• Water Cycle Mat
• Rock Cycle Mat
• Coral Reef Habitat Mat
• Everglades Habitat Mat

This particular card in question is from the Coral Reef Habitat Mat:

After children make sentences with the cards, they can write and symbolize them with the stencil. Last school year, a child was doing this work and he symbolized "underwater" as a noun in a sentence where it clearly was an adjective! So I pulled this card out to think about it some more. I can see sentences where "underwater" is an adjective and an adverb (of location), but I can't think of a single example where "underwater" is actually a noun. I don't think this card should be black!

I swam underwater to the sunken treasure chest. (adv)

The underwater robot got lost during its exploration of the Titanic. (adj)

Can anybody think of one?