If I'm not mistaken, swimwear is בגד ים - how does it become בגד הים in this sentence?
You don't have "The" in the English sentence but it is implied that we are taliking about a specific swimsuit,
The swimwear = בגד הים
This sentence can't work in Hebrew without the ה
Why not הבגד הים instead?
I believe it's because in this "construct" state
(e.g. noun-of noun) the whole phrase is treated as a single unit, and the last noun is the only one that takes the definite article. This is a very common structure in Hebrew.
In "real life," on the street, would you typically hear "זה בגד הים שלך" or "זה הבגד-ים שלך" or both?
Is בגדי ים not acceptable?
Is בגדי הים accepted here for "the swimwear" , or is it only בגד הים if swimwear is always singular ?
I think that, in addition to the construct form, the possessive pronoun שלך makes the noun definite, therefore the article.