Get a bucket and run some water out of the drain of the WH and see what it looks like. Also keep in mind that if any piping in the house is galvanized iron, as it ages, it will rust as well. These were somewhat common on pipe nipples for tub spouts, and could be anywhere in the house including the main supply piping. Hot will possibly cause it to rust quicker than on the cold. If you had significant rust from inside of the WH, it would probably be leaking by now.