SD is Swords Dance, DD is Dragon Dance. I figure Feraligatr is slow enough that even if he does get an SD off, he could very well get killed the next turn before he can move. DD helps fix that speed problem right up. He'll do less damage, obviously, but it's probably better to do less damage than to do none because you went down before you could do anything.
I say dump Absol for Mamoswine, but the decision's up to you of course. Absol's job is basically to have a nice attack stat and to use priority moves, and Mamoswine does that too. Just Mamoswine is faster, has a better type on his priority, and has a better movepool.
(You really shoulda been able to work out SD on your own, by the way. Not like there are a lot of moves on your Feraligatr that have the letters S and D in them, are there?)