Noun, but it can be a gerund or participle.
"Synchronized swimming" is a phrase. The whole phrase together is used as a noun. The word "synchronized" is an adjective. The word "swimming" is, when used like this, a noun.
The word "seemed" in this sentence is a verb. Specifically, it is the linking verb that connects the subject "swimming in the pool" with the predicate "desirable."
It is an adverb for example- "He ran immediately after swimming in the Iron Man race."
part of speech
The part of speech for this particular word is a noun.
adverb
what part of speech is beneath
The part of speech for "answer" is a noun.
The part of speech for "explicit" is an adjective.
The part of speech for "unfamiliar" is an adjective.
The part of speech for "indefinite" is an adjective.