Just back from a week in Zante with my son (10 years old).  Booked the glass bottom boat trip in Tsilivi for 28 Euros for us both; good value I reckon for an afternoon (pick up at 1.30pm, back to Tsilivi at 6pmish).  Great because there weren't too many people on the boat - we saw a turtle not long after leaving the harbour.  Plenty of time for swimming too.  My only groan is.... Please folks don't throw ciggy butts into the water!!  Surely this lovely environment is worth protecting?