One could also say that, if one can pay to have all one's shopping delivered (or even get it delivered for free), as one can with all the supermarkets, businesses that have an on-line shop as well as a physical shop, etc., it is not essential for the vast majority of the population to go to Tesco, Waitrose etc., but that activity hasn't been prohibited, or even questioned.
The legislation (enforceable) only says, "No person...may leave or be outside of the place where they are living without reasonable excuse." It sets no limit on the distance that may be travelled, nor does it require the travel to be essential.
The guidance (unenforceable) says, "...If you need to travel you should stay local – meaning avoiding travelling outside of your village, town or the part of a city where you live..."
I was offering the OP a possible answer to his problem. It would be for him to decide whether, in his particular circumstances, that answer is appropriate or not. Since he has now revealed he would have a 4.5 hr drive to get to his boat, I wouldn't say it was appropriate, but it might be for someone living closer to their boat, facing the same situation