Why "out to sea"?
We use the word "to" to indicate a direction or progress in a direction. We say "just to your left" or "far to the north", to indicate places in those directions. "Just out to sea", then, means a little way in that direction from the water's edge.
If they bobbed just out of [the] sea they would not be in the sea, in fact without the the phrase seems grammatically incorrect.
Out to sea means they are in or on the water. Just out to sea means they are not far off the coast.