Police said the mortar shells with registration codes written in Thai were in a perfectly usable condition. Initial examinations indicated they were produced in Thailand.
The cache was brought to Bangkok for examination.
Local police said the ammunition and firearm might belong to influential groups, or arms dealers in the area.
Police said the owner may have come across a police checkpoint while delivering the munitions and been forced to hide it under water.
The ammunition and the firearm were not old and were likely to have been dumped in the pond about a month or two ago, according to police.