Don't ask how long Sail Forth lasts. Don't check out its average playtime. Just give yourself to it, ten minutes here and there, and let it become a watery companion through your days and nights, the sun-baked memories of it lapping at your dreams and waking thoughts.
Sail Forth is basically the parts of The Wind-Waker when you were on a boat bombing between islands. You have the whole of the ocean to explore, and it's filled with stuff to do. You can shoot baddies with your cannon, collect scrap to sell, chat to people on islands and chat with passing boats - I think in Melville's day this was referred to as a "gam", and often included an exchange of mail.
What else? Onwards and outwards, I think: you can hire new sailors, buy new and bigger boats, deck them out with flashier weapons. That's fine. The Katamari power curve is always waiting. I prefer to ignore all that, though. I find the camera - another Wind-Waker nod - and take pictures. I chat to people and see what they want, learn what they value.
More than anything I watch the waves roll up and beneath me, watch the sky turn all the bruised colours of the evening and dawn, and I enjoy gadding about from one place to the next.
It's busywork - undeniably so - but when the busywork is dressed up as meeting pirates and finding new parts of a sea chart I'm all for it. One of many quiet things Sail Forth has learned from Zelda was there in the very first Zelda game, too: it hints at things you aren't ready to do yet. You'll find a clue to a part of the sea you can't reach. You'll find a resource that doesn't make much sense given what you currently know.
A friend described Sail Forth to me as a kind of watery Outer Wilds - its own self-contained universe with its own rules and leylines. That hasn't clicked into place for me yet, but it's still early days. Just last night I passed a lighthouse where nobody was home, and then I learned later on that the lighthouse's occupants are nocturnal. Was my first visit during the day? I can't remember. But I'm going to head back tonight and see what I can see.