What motivated him? He wants to sit by the water, which seems quite logical to me. This has nothing to do with you. He neither wants to annoy you nor does he know the building regulations.
Regarding the enclosure: If you provoke him, he will put up a 50cm hunter's fence and that’s that. Just build a fence yourself. A solid one, e.g., made of wood, not a hedge, because otherwise the next disputes are preprogrammed, since the hedge grows and your neighbor demands that you trim both sides.
Semi off-topic: I recently witnessed such a hedge dispute in my circle of acquaintances. Friends of ours demanded that the neighbor trim the hedge on their side. Result: the hedge was replaced by a wire mesh fence. Now they are even more annoyed with the neighbor, especially because with only 8m of property width EVERYTHING has really become visible A new hedge must be planted 50cm away from the common boundary.