As a test... take your exported elevation bit map and make a black border around the edges of the file RGB value of 0,0,0 about 2 pixels wide.
Then re import your elevation bit map and see if that helps.
Often times I find that there are terrain cells at the edge of the map that are actually land, but the slope of it makes the
water that is just off the map, have a slope of greater than 6ft per mile.
I'm not saying that this is your problem but it could be.
midi
P.S. oh and make sure you save then re open the .oba file after importing the new bit map.