So here's the problem: right now most islands in the ocean can't be covered by a realm, because they are too far from claimed land. Even by building cairns right on the shore, you can't reach the island's shores to build another one and claim the place.
I see two possible solutions to this:
1. A simple solution would be make cairns claim a larger area. I don't really like this idea, because of how it would affect realms on continents, but it would work for islands.
2. Add a special kind of cairn that can be built outside realm territory, at a certain range. This would allow realms to claim all islands next to them, without letting them to claim far away land. A kind of "outpost" claim, or something of the sort.
To be clear, nobody is asking for every little piece of sand to be claimable. I'm talking about mid-large islands that are reachable through sea water (the deep water that doesn't break rowboats to be clear) but are often outside cairn range from shore.
PS. Before anyone says "build a new realm on that island". With the introduction of monolith and Irminsul as requisite for several buffs, that would't be effective. The island realm couldn't reach land to claim those, so that's not a valuable solution.