My understanding is you will oscillate up and down as you fall through the gravity plane, slow, reverse then go through the gravity plane again, reverse, and so on. Eventually you are just floating at the level of the gravity plane.
Since there is no ground to walk on or water to swim in you just float if you cannot fly. It is fairly easy to throw a rescue rope to someone just floating from where they fell though.
Significantly, this gravity plane is two-directional; it attracts from both top and bottom. A sailor can actually stand on the bottom of the ship 's hull and move around as easily as if he was walking on deck. In this case, "down" is actually "up," back toward the plane of gravity that cuts through the ship.
One of the stranger side effects of all this is that an object falling off the side of a spelljammer can oscillate back and forth across the plane of gravity, falling first in one direction until it crosses the plane , then reversing direction and falling back across the plane again, and so on until something causes it to stop. To a person standing on the deck, the object appears to fall down, then up, then down, then up. This trick is commonly used to amuse passengers new to space travel. More than one groundling has gotten in trouble for standing at the ship 's rail and tossing an endless stream of apples over board just to watch them bob.
Example: A viking longboat cruising through the fjords is suddenly thrown from its home into wildspace. Its crew can still stand normally on its deck. Down for them is toward the bottom of the boat. Similarly, an individual could stand on the bottom of the longship, and find that "down" for him was back toward the top of the boat. Someone who jumps off the deck of the boat would fall downward and pass through the boat's plane of gravity, whereupon the direction of gravity would reverse for him and pull him back toward the underside of the hull.
A carefully-thrown object can actually go into orbit around the ship. Such orbits do not last long; the object quickly collides with the ship's rigging or oars, or slows down in the air envelope and crashes into the ship. One of the favorite pranks of experienced sailors is to toss an apple or similar light object off one side of a ship in such a way that it curves around and strikes someone standing at the other rail.
And page 13:
There is a problem for unrestrained objects resting on the plane of gravity of another large object, however. Along the plane an object is weightless, but it is pushed slowly out toward the edge of the gravity field. Therefore, a man overboard would eventually come to rest at the ship's plane of gravity, then begin drifting away from the ship along that plane toward the edge of the air envelope. On reaching the end of the gravity plane he is pushed outside the air envelope and then left behind as the ship moves away.
Because objects at the level of the gravity plane itself are weightless and undergoing a small but definite push toward the edges, this center plane is often used to launch heavy missiles or prepare boarders to attack other ships.
Aside from this slight push, there is no relative motion of a ship within its air envelope, aside from turning. A ship's air envelope does not turn with the ship when it turns, but objects in the ship's air envelope do not drift toward the rear of the ship simply because the ship is moving forward.