In the quiz at which I was officiating, I was not able to accept the first answer given, which (IIRC) was "the North Sea". The person concerned was not very happy about this, but Wikipedia does describe Bass Rock as "an island in the outer part of the Firth of Forth".
If you look at the map, Bass Rock is some distance to the west (i.e. the mainland or upstream side) of a line drawn due south from the easternmost headland on the north shore of the Firth. I'm no expert, but I think I would agree that this puts it in the Firth. Not only this, but the Isle of May is slightly to the east of that line, and according to Wikipedia even this is in the (outer) Firth.
If you also take into account the fact that the question asked for "a stretch of British water" (my italics), I would suggest that this rules out the North Sea. So on this occasion, I think the question setter has got it right – even if it might have been helpful to give a further clue in the wording of the question. For example: "The Bass Rock is in the outer part of which British river estuary?"
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