I might have a possible explanation
If you take a close look at this picture, you'll see small stairs at the bow (and a Santa hat on the figurehead)here
You can also see them on this model ship
On HMS Trincomalee
there are 2 gunport holes and a gap in the railings.
Both of these ships have the toilets at the usual place on the bow.
Somehow I can't reply at the moment so I'm just gonna edit this post:
kurigan, on 13 March 2012 - 07:55 PM, said:
but on smaller ships, like any of those "Surprises", where there isn't space for the convenient seat, raise the question, where did the common seamen take their more urgent business?
In the case of HMS Surprise I can still give a clear answer.
As you can see here
there are no toilets at the bow, but a sailor would still use the same spot.
You can see this in a scene from the well-known movie here:
However, I don't know the answer for real small ships like sloops.
Guess it would just go over the railings.