I found out in the Imperial War Museum, London that not so careful submariners, if using the toilet incorrectly, could get a back surge and end up covered in their own mess, apparently leading the the term 'getting your own back'. Anybody else heard of this?
I am an ex-submariner but this refers to the diesel boats from way back when up to about the late 60s, early seventies. And yes it could happen. Now as to whether the saying actually originates from this is a moot point. It was certainly used for this event but I seem to remember it being possibly attributed to way before then when using the over the side 'heads' on sailing ships in rough weather.
The earliest recorded use of the phrase, in the form 'get his own back' - meaning 'revenge' - appeared in 1910 and was a reference to one of the opponents in a boxing-match. Certainly submarines existed before that, but I suspect they were not the original source of the phrase.