Hi,
HIGH WATER MARK indicates the level upto which the last table block reached.As soon as u enter data into the table HIGH WATER MARK also rises.When u delete the rows from the table the HIGH WATER MARK remains where it is,it doesnot fall.This leads to more I/O since when Oracle performs scan of the table it scans upto the HIGH WATER MARK.
HIGH WATER MARK is reset only when u truncate or export the table,drop the table and import it again.
In case of any help please be free to ask me at [email protected]

Regards,
Rohit nirkhe,Oracle DBA,OCP 8i
[email protected]