The two Thais were a tourist guide and a boatman who took the group to the Nam Talu cave, which is 500m deep, on the Rachaprpa reservoir.
Police Lieutenant-Colonel Pichan Kalayasiri said: "We have found eight bodies so far, including six foreigners and two Thais.
"The dead included a 10-year-old German boy, but officials still cannot tell the nationalities of the three women and two men."
Colonel Akom Saisamai, a local police chief, said the tourists were not carrying their passports, hampering the identification process.
Local television showed images of rescuers carrying body bags from a boat overnight and laying them out on the shore.
Kao Sok National Park covers two inland districts in Surat Thani.
Its large lake, thick rainforest, limestone cliffs and numerous caves make it a popular destination for trekkers.
According to the Surant Thani provincial tourism website, 20,830 tourists visited the park in 2006.
However, Thailand's tourism industry took another hit last month when 54 foreigners and 36 Thais died in a plane crash on the resort island of Phuket.