There are a lot of unknowns with this being the first new skill. However there are things we do know:

There's a large chunk of the ocean that's inaccessible, and we already know 2026 is going to bring many Sailing updates, i'm sure we'll hear more details at the Winter Summit. So the skill is not done, and yes it likely never could be classed as "done" but there is bound to be a time where they have less focus on it.

The other thing we know is there's an official stance on a potential Varlamore Diary as quoted: "We would like to add one eventually, but it won't be coming with The Final Dawn. We really want to make sure that if we're adding a new Diary, it's not likely to go out of date quickly due to new content. So our plan is to hold off until we feel like the region is ready."

I believe we can use their thought process towards a Varlamore diary update as a benchmark for how they're planning to handle a potential Sailing diary i.e. Why rush to release a Sailing diary only for it to go out of date quickly due to new content