Ayothaya Floating Market is not an ancient market that has simply survived unchanged; it is a purpose-built visitor attraction. Taken on those terms, it can still be enjoyable, with canals, food stalls, souvenir shops, and cultural performances packaged for Thai and foreign tourists.
Go if you want an easy, family-friendly break between temple ruins, not if you are looking for raw local commerce. The best approach is to snack, wander, and enjoy the stage-managed atmosphere without mistaking it for old Ayutthaya itself.