The DST-700 motors work surprisingly well in salt water, even with the original carbon steel bearings. As long as you give the vehicle a good fresh-water rinse after each dive, and hit the bearings with some WD-40 or other lube, you'll do perfectly fine.
If you're in a situation where you can't rinse the vehicle after a saltwater dive, then upgrading the bearings would be appropriate.
The DST-700 motors are nice in that the front and rear bearings are the same size, unlike the earlier 2213 motors used in the 2.6 and earlier vehicles. Offhand I don't know what size they are, since I've never felt the need to upgrade the bearings on any of my ROVs.
If you do upgrade your bearings, please post some pictures and information on how you did it, and what kind of results you achieved.