We have similar easements and accesses in my neck of the woods. One of the most contentious where I live is public access to lakes (I gather this is also an issue in Hawaii with access to beaches). Basically the law says that landowners are certainly allowed to own land up to the beach, but they cannot own the beach or any stretch of the water. There are some slight variances on this principle for self-contained bodies of water, like artificial lakes, but in general, you can own land adjacent to a lake or stream, but you don't own the lake or stream, or the immediate vicinity around it. Further, there are public access points to the beach, which often do cross peoples' property, but by law the property owners cannot impede peoples' access to the lake, nor can they attempt to block the access points. Further, if they build warfs or boat launches, well, they're doing so on public land, so while they may be free to locate those structures there, they can't prevent other people from using them.
Every year property owners around various lakes in the area try to block access trails, make absurd threats against people enjoying what constitute public lands, and generally be fucking assholes. That they bought this land knowing full well that they are not lawfully empower to prevent access is irrelevant. They're big crybabies who want to assert defacto ownership over land and water that explicitly does not belong to them, and never will.