I haven't tried mine in salt water so I can't really say one way or the other.
If you visit Whites forums there's a good post (from deolslyfox) about the Sport and his experience using it in salt water. As I recall he said it will work but the sensitivity had to be turned down.
I'm a fan of Whites but probably wouldn't choose the Sport if I did 90% of my hunting in salt water. I'd look at the BH 300 (no longer in production, but you can find dealers -forum sponsors- who have one in stock) if the ability to discriminate trash is important. Were I a dig it all person who did 90% of my hunting in salt water, I'd look at PI machines (Whites SurfMaster DF or Tesoro Sand Shark would be the two I'd really look at).
As far as costs go, I believe the Tesoro Sand Shark is less money than the MX Sport. I think the BH 300 and the SurfMaster DF would be roughly $100 more than the MX Sport. Call the forum sponsors to find their best price.