farraige = sea
muir = sea
aigéan = ocean
Although Ó Dónaill also gives an fharraige mhór and an mhuir mhór for both "the open sea" and "the ocean".
The phrase ar muir is well attested for "at sea" in Irish. Alternatives include ar farraige and ar an bhfarraige.
I think the translation of san fharraige as "in the sea" given here is both accurate and reasonable. If they wanted to say "at sea", they wouldn't have used san fharraige in Irish.
If they wanted to say "at sea", they wouldn't have used san fharraige in Irish.
"at sea" is not a reasonable translation, because the person who created this exercise had the option of saying Úsáidtear fomhuireáin ar an bhfarraige, but they made the deliberate choice not to.