Parasite Pig — William Sleator

Barney's stuck at a boring after-school job, earning money to repay his parents after their beach house is destroyed in a battle with aliens.  Of course, they don't believe that aliens did the damage.  No one, in fact, realizes that sixteen-year-old Barney saved the world by outsmarting the visitors at their violent game, saved the world by outsmarting the visitors at their violent game, Interstellar Pig.  No one but the aliens-and for them the game is far from over.  Barney is about to become the unwilling partner of a chatty intestinal parasite; the potential snack of giant, man-eating crabs; and the competitor of a stinger-happy seven-foot wasp woman.  Life just got a lot more interesting...