those ruins are at xunantunich, near the guatemalan border and benque viejo...

from AC, its easier to get to Lamanai or Altun Ha. any of the folks at:
//AmbergrisCaye.com/tours/

can get you there!

definently have to go to the mainland to see the large ruins. Altun Ha is a shorter trip, Lamanai is longer, and you see a bit more river life.. Lamanai is an all day trip.