Caravan sites and so on are explicitly required to remain closed in the regulations so those have to stay closed.As far as I know there is no published guidance permitting staying on board boats, or caravans, or tents. these are not mentioned in guidance I've seen. In fact it still requires us to minimise time spent away from home, with work, medical issues and exercise as specific exceptions. It specifically prevents us staying in other homes and holiday homes.
But as a favour to me, could you show me the part of the guidance or regulations that makes you think that we can't stay in holiday homes, and must minimise time outside the home, but can stay in boats, caravans, tents etc?
The regulations do no require us to minimise the time spent away from home - on the contrary the advice associated with the latest revision says you can spend as much time as you like, and travel as far as you like, to pursue your outdoor pursuits.
I can't show you where the regulations permit anything mainly because in UK law everything is lawful unless it is explicitly declared unlawful. Going to a boat is well understood to be an acceptable under 6(b) and nothing restricts the amount of time you can spend on that recreation. It is universally acknowledged that night fishing is allowed so there is no doubt that sailing overnight is also permitted.
Whether staying on the boat overnight without sailing is allowed is a slightly gray area but there is nothing in the regulation that could be used to say that and nothing in the latest round of advice that implies it.
Note that the regulations don't explicitly forbid staying overnight in holiday homes (other than caravans that are prohibited) so if you could argue that you had a good reason for being at your home (perhaps to take exercise in the grounds) then you could argue it was reasonable. However that would be hard to do given the advice (not regulations) says one should not.
As far as the inland waterways are concerned the advice from the Canal and Riverboat trust is that staying on your boat on your home moorings is allowed
But let's not forget the ONLY reason for the regulations is to prevent the spread of the coronavirus. If you think anchoring is more likely to spread the virus than driving home then it is your moral duty not to do so even if it is lawful. If you don't then you should feel free to do anything not explicitly banned.