Poor old Canute. In apparently seeking merely to demonstrate to sycophantic courtiers the limits of his powers he has been remembered for an act of preposterous hubris: ordering the waves to retreat. The proper lesson from this tale ought not be that one cannot hold the sea at bay. The sea can indeed be held at bay by Dikes or Levees, otherwise Holland, for example, wouldn't exist. One key feature of a dike is that it is an all or nothing solution. It would be an act of pointlessness equivalent to the popular version of Canute's command to build half a dike - the sea would simply engulf it around the incomplete portion.
Imagine a curved bay, subject to coastal erosion, composed of adjoining strips of property perpendicular to the shore. Let's say there are 50 landowners and a majority agree that coastal erosion is a bad thing and a proper solution to the problem would be to build a sea wall, the entire length of the bay with each landowner responsible for his or her own portion. As I note above, the sea wall is all or nothing. The landowners who do build their walls might as well take the money it cost to build them put it in a big oil drum and burn it for all the good it would do, if the remainder don't also complete their walls.
A sea wall is an expensive bit of construction, let's say some landowners are poorer than the others and have more pressing needs than building a seawall and, further, rely on the eroded coast for fishing. It's not much consolation as you lie starving to know that in the future the full extent of your land will be protected from the sea. We could also posit that there are other landowners who are rich enough to afford the wall but aren't sufficiently bothered by the rate of coastal erosion to spend the money on the wall, perhaps they plan to incorporate a marina in the portion of their land projected to be engulfed.
It should be clear that the only way that coastal erosion may be forestalled in this scenario is a) for there to be unanimous agreement that this is a good thing and worth the cost and b) for the wall to be continuous and complete. There is no point in one "virtuous" landowner "setting an example" to his peers by constructing his wall ahead of schedule - even if he does manage to convince a large number of landowners to follow his example - nor is there any point in excusing poorer landowners from building the wall. It either gets built or it doesn't and, if it doesn't get built, any effort or expense on construction of a partial wall is a) a waste of money and b) distracts from adjusting to the eventuality of coastal erosion, particularly by engendering a false sense of "doing something".
Even assuming everything that is said by activists about the cause of climate change is correct, it's still going to be a mistake to follow their policy prescriptions unless and until there is a) consensus agreement that this is desirable and worth the cost and b) some method to ensure full compliance by everyone. I see no evidence for a) or b).
[Edit: based on a comment from Jon below, posit that the proposed sea wall is a line of defence a bit out into the sea rather than on or behind the current high water mark]