5.10 If I use Query and find/delete all entries in a dataset, is the high water mark (HWM) set to zero?

If you are asking about a master dataset then the HWM is always set to the capacity of the master.

If you are asking about a detail dataset then when the last entry is deleted and the FreeEntryCount is equal to the capacity, DBDELETE will reset the HighWaterMark and the deleteChainHead to zero.