Spheres that regularly use airship travel know of their Sphere's gravity threshold, and prepare for the [i]immediate[/i] shift in gravity when they approach that threshold (which is a specific height distance from the Sphere surface). Depending on how they're oriented to the Ocean, the passengers lock themselves in place, the ship passes the threshold, and it orients itself so that it's bottom is pointed to the Ocean.

It's a sudden change with no gradual transition.