It definately sounds like the dip tube is shot. There are ways to replace it. If I'm right, what's happening is the cold water coming in, instead of being directed to the bottom of the tank by the dip tube, is showing up right at the top, diluting the hot which normally would rise to the top.
There was a big problem with DIP tubes about 10-years ago, so unless yours was old stock, it is somewhat unusual for it to go bad. If you have flexible lines feeding the WH, you may be able to replace the dip tube yourself. Otherwise, it would require some cutting and rebuilding the supply piping. One company makes a nipple with a dip tube in it, if yours can't take a more conventional one. Note, this assumes you have enough height above the WH to install it. If not, then you'd have to disconnect the whole thing, drain it, tip it over so you'd have enough room to insert the new dip tube. While you have that, you might as well replace the anode rod. If people replaced their anode rods periodically, the tank should last much longer. Note, if it comes to that, you might consider just replacing the whole thing since the labor is nearly the same as installing a new one. You might call the manufacturer to find out what your options are since it should still be under warranty (normally parts only, a very small part of the cost to repair).