I had no idea before, but I got it thanks to the clue! HOH is water! The letters "PIASA" correspond to the 5 oceans: Pacific, Indian, Atlantic, Southern and Arctic. The Pacific has an average depth of 4000-30=3970 metres; the Indian has an average depth of 4000-30-229=3741 metres; down to the Arctic which has a depth of 4000-30-229-95-376-2065=1205 metres. Very nice challenge!
What can I say mushroomsteve, brilliantly solved. I could not have explained it better... very impressed. Definitely worth a Challenge Gold Medal. Your second CGM.......... :o)