It's a bit more complicated (as always). I'll try to outline, but you'll understand that I don't want to give away the location!
The house we live in predates the marina itself. It was originally a warehouse for the river and has been all sorts of things over the past 300 years or so. The building is now a number of private houses, including some added as new builds 20 years ago. Until recently, the marina and houses were all owned by the same people. But now it's all been sold off. The houses were sold individually, but collectively own some land. This is mainly the road on the front, but also includes a tiny strip at the back adjacent to the marina, which has also been sold to someone else.
The fact that it was all owned by the same family for a lot over 60 years means the paperwork is a bit shoddy as it was never intended to be separate parts. However, our Land Registry title shows that we own a bit of the path in the marina - they've drawn a straight line from the end of the gardens, while the bank actually curves inwards into our bit. So basically, we (collectively, not as an individual house) own the bit that you'd actually moor on. Do we own the water? Very doubtful and as Machpoint005 says, very expensive to contest.
Now, we do have other arguments with the owners (for example, we can park there - our deeds say so, but are we allowed to drive through to the back of our houses? They say no, we says yes because right of way has been established for well over the 20 years needed for it to be default and he'd have to go to court to have that rescinded) but I don't think "We own that tiny bit of water" is one we want to get into.
The question really is "are we allowed to take a boat out to the river?". There is nothing in our deeds that says so. Firstly, because the deeds for the houses were written when the whole thing was owned by the same person, and he didn't give any specific marina rights. Secondly, because the land that we own was only drawn up last year when they hived off the houses to sell. The deeds massively predate the house coming with any external land beyond the garden and it's a bit of an accident that we find ourselves with some bank. I've done some digging and there doesn't seem to be right of access to water the same way that there is for the road.
So, as I say, we're willing to pay a reasonable sum for him to allow us to take a boat out, through marina that he clearly owns. And he has started charging everyone for access on an ad-hoc basis. But what would be reasonable?!
No, the mooring is in an entirely man-made bit. However, he's exactly the sort of chap who would put the debt collectors on you if you didn't pay up.
Although he hasn't yet posted any actually fees anywhere and we've not agreed to anything.