The only times DD isn't great against Enchantress are when the player has a huge head start (permanents all over the table, maxed-out hand, etc.) and is sandbagging land and enchantresses to re-establish the board in anticipation of DD, or if he has all Presences but no Argothians. Even then, I'd probably still do it just to provide a speed bump and eat his Wild Growths.
If there's only one Enchantress player in your area, I don't know if you want to bother hating the deck out. It depends on how likely you are to get paired against Enchantress and how precious the rest of your maindeck and/or sideboard space is, and whether DD (or whatever) would be useful against other decks in your meta. Harmonic Convergence is ridiculously narrow, so you probably want to throw that out unless you have space to burn and want to surprise that Enchantress player. Reverent Silence is mainly for Leyline of the Void, but it's obviously strong against a deck full of enchantments. Other cards that are good against Enchantress and could be good elsewhere are Engineered Explosives, Firespout, Perish and Krosan Grip.
I play in a blue-based meta and haven't had problems with DD, but I run Wishes, so it takes up just one slot. If you don't have Wishes, you have to structure the deck around it more. (Micah Greenbaum's 4C list [http://sales.starcitygames.com/deckd...?DeckID=39297] is an example of that approach, although he runs some Wishes too.). Just because your opponent is blue doesn't mean he's always going to have cards in hand. Anyone without a hand is vulnerable to Wish --> DD, which is usually game. But, yeah, if you're not careful with it, you can get burned. And, sure, Wishes can get Snared, but so can Bob. Feel free to run something else in that slot if you're not happy with it; just make sure it gives you as much overall as the Wishes give you.