To me there are two questions here:
1. Is it sensible and
2. Is it legal.
Both involve a recognition that the government is not really interested in you as an individual but rather about protecting the NHS and ultimately the economy. No sane regulation can envisage every obscure situation.
In the present situation it seems to me that the overriding objective is to restrict an individual meeting, in close proximity, with anyone outside their family – and even within the family with limits on group size. As has been observed solitary outdoor pursuits are being encouraged for both physical health and mental well being with the example being fishing.
Personally I see no Covid difference, morally, legally, or disease wise between walking around a pond, sitting beside it fishing or indeed sailing my model boat upon it. All have the proviso that I must maintain good social distancing.
Where it gets a but grey legally is if I sail on a private club water at a normal club sailing time. This must be a problem shared with fishing clubs who rent private lakes and the commercial fishing Carp lakes that we have around here.
What concerns me is that we are going to waste any gains from this lockdown in December with scrambles to the shops, pubs, family parties etc. just as we did with the mad few days before the lockdown started. Some elements of society seem much more concerned by what they can get away with than what is sensible.
I am being very careful, and hopefully sensible. I don't see a real problem with the answers to either of my questions.
NOTICE: I am not a lawyer and this is my probably poorly informed private opinion which nobody should rely on for any purpose whatsoever!!!