High and low of it

David was strolling along the harbor one late evening when he spotted a deep sea fishing boat in the harbor.

Over its side he saw a rope ladder hanging with its end just touching the water. Rungs of the ladder were one foot apart. since it was a full moon night , the tide started rising. the tide rose at the rate of eight inches an hour.

David stood wondering at the fast rate of the tide and started calculating how many of the rungs will be covered  by mid night exactly at the end of six hours?