But if you type 'The fish is swimming about you' it says incorrect answer though both under and about are given as options.
I think it only means 'about' in the sense of 'regarding' (e.g. 'I emailed you about the meeting')
I translated this as 'the fish are swimming around you'. Is that not an acceptable translation?
If you want to translate 'around something' into Irish, you can use the adverb 'timpeall' followed by the genitive form of the noun. There are other ways of translating it, but I'm afraid that 'faoi' isn't one of them.
Faoi gives more of a meaning of "about", like something is about something, so under really is the best one here.
'Tá iasc ag snámh fuibh' should do the trick.
'An t-iasc' means "the fish" because 'an' is the definite article, "the". Irish doesn't have the indefinite article "a", so if you want to say "a fish" you just say 'iasc'. E.g. 'caithim gúna' means "I wear a dress" rather than "I wear dress".
Out of curiosity, in what situation would your even be in where there was a fish swimming under you other than in open water?
The preposition "ag" (at) is used with a verb in Irish to create the English equivalent of a "gerund"--the "-ing" form of any verb. You could think of it as being "at" doing something--"tá ag snámh" is "being at swimming" but the translation just uses the English gerund.