noun
swim trunks; swimming trunks
Common term for men's swimwear, especially the shorts style. Often used for beach or pool wear.
新しい海水パンツを買って海に行った。
I bought new swim trunks and went to the beach.
水着 is the general term for swimwear (for any gender), while 海水パンツ specifically refers to men's swim trunks.
Compound of 海水 (seawater) and パンツ (pants/underwear), literally 'seawater pants'.