As an owner of an alleged gin palace I can confirm they do indeed exist and they are covered under the notice to mariners (very rarely read by recreational mariners) AND more importantly you are expected to call them up on radio if going near them - marked on charts.
It IS true that if you dont contact them and they fail to contact you they WILL stop firing - and they get rather p.ss.d. They will often have many patrol vessels around an active danger area.
It is just like flying, they get there fair share of "busts" and are more active in alerting you by radio or patrol vessel. Many coastal mariners (=local bimbler) will often not even have charts
I was once called up by them 5nm outside their area just in case I got too close - they were very nice about it, and in fact once they found out my destination (the other side of them) they allowed me to route straight thru (i think they wanted lunch)!
Places like lulworth cove can get very busy with visting vessels and the patrol boats can be kept very active.
An interesting point is that most radio equipped boats listen out on a common frequency (ch 16) so can easily be addressed when going near a zone - not quite so easy given the frequencies used on a plane.
Another interesting point is that there seems to be little sanction since most recreational mariners dont have a license to loose. I witnessed a vessel going withing 10 metres of a submarine (near plymouth exercises) - a very illegal moving danger zone - they attempted to radio him , got very aggressive because he did not reply they then submerged to avoid any conflict - he heard nothing more (so he says).
Remember most boats can typically do < 40kts (sailors < 10kts) and radar works well on the sea - so they get plenty of notice.