Hmm I dunno about that. I mean, if it were a world-spanning wall of land, sure, it would sift everything out of the sea sooner or later, but a lil’ island?
Ocean currents have a semi-permanent geography even in the real world (which is how you get doldrums and “trash vortices” and so on), and it seems like a world with an uninterrupted fluid surface would, like Jupiter and other gas giants, be made up of stable coriolis bands. So if you were more or less drifting, you’d just spend your whole life going round and round at the same latitude.
You could sail North or South on purpose, but (a) why, if there’s nowhere to go and (b) by the same reasoning there would be horse latitudes where you could get stuck and die. The movie does claim that a few people (Dennis Hopper & the Pittsburgh Steelers) still have fossil fuels lol, but even so, why would they venture into areas where there’s no one else around to pillage? So I can totally buy that Dry Land is never discovered by accident.
That said, if it’s not somewhere that you’d run into it by accident, it’s hard to believe anyone could find it on purpose, either. On a world with no landmarks, no satellites, and no good technology, it would be very hard to navigate to a precise longitude. Iirc the “map” in the movie is just some CJK characters tattooed on a minor for some reason, so at least they had the sense to be cryptic about how that’s supposed to work.
The single dumbest bit of worldbuilding is the bad guys having all that oil when no one’s seen land for generations. And apparently the only people who’ve managed to conserve their supply this long are the sort of people who basically roll coal on the ocean, using priceless diesel to brush their teeth & what not. What kind of environmental message is that?
And another thing! What, in this scenario, is so great about Dry Land anyway? People live on boats now. Why exactly are we so happy that an elite minority gets to cosplay a pastiche of the distant past? The whole thing is basically a thinly-veiled metaphor for middle-class white people fantasising about moving to New Zealand.