@Delpf
"Sea toilet is not legal in the UK (but is in France, I believe)."
You have this the wrong way round. In France, new boats built since 2008 are required to have a holding tank, or something similar, and older boats that don't have a holding tank are not allowed to discharge black water in marinas, rivers or canals (but many French canal barges disregard this).
In the UK there is no general law against releasing black water. In some areas, such as those which include marinas, there may be local byelaws that prohibit doing this, but otherwise, you're free to do as you like in most areas of the UK.
Also, marinas usually have rules that prohibit this. It is likely this reason why the OP was faced with this dilemma.
For example, on the rivers Trent, Ouse and Nene (they're in my part of the country) you can use a sea toilet just about anywhere you like. In contrast, there is a Port of London byelaw that prevents commercial vessels and houseboats doing this on the Thames but does not apply to pleasure vessels.
So, if you have your own motorboat, yacht, or canal barge (as long as you don't actually live on it full time) then you are free to empty out your black water right under Tower Bridge if you so wan tot.
Just as an aside, in American waters it is illegal to perform in a bucket then tip it over the side but it is ok to do it directly into the sea! But, other than that, they are pretty strict about not releasing black water within three miles of the coast.