The word "floating" is used to describe the rather mixed feelings that the narrator has as she is taken away from her home country to a new country that is foreign to her and where she has to struggle to belong. Note the following quote:
...and instead of sinking down as I'd always done,
magically, that night, I could stay up,
floating out, past the driveway, past the gates,
in the black Ford, Papi grim at the wheel,
winding through back roads, stroke by difficult stroke,
out on the highway, heading toward the coast.
Swimming is clearly an extended metaphor that is used in this poem to describe the experience of leaving the narrator's homeland, the Dominican Republic. Finally, and rather ironically, the narrator is able to float for the first time, but only as they leave her beloved homeland. This of course reflects the narrator's understanding of what is happening to her and the tremendous loss it is both to her and all of her family as they are forced to flee their home country as political refugees.