well... maybe its cuz the armor? :D... have u ever tried to swim with ur clothes on? its heavy. If u ever see someone drowning, atleast take ur shoes off, u cannot imagine how hard it is to swim when ur boots are filled with water. :P its funny to see how many videogames allow the player to swim despite the fact that they wear some heavyepixmultidragonkillaahrmour :D.
Don't tell Bungie I told you this, but all Spartans are secretly cats. You know, cats hate water. That's why they can't swim! And by the way, the Master Chief's name is really John, it's Whiskers-117.