after entering water end dragon doesn't get hurt

(in creative)i was experimenting in the end and i placed water down, it flowed off the map in three streams and the ender flew threw it. hateing the enderdragon i tried to kill it w/diamond sharpness 100 (commands) but it wouldn't take damage. i tried this three times, befor it went in watter i could hit it, then after i couldn't hit it