when dealing with S30V/M2/D2/etc i like the DMT diamond stones with a few drops of water, i have a folding DMT with fine on 1 side and coarse on the other if ya free hand sharpen this is an excellent stone to have, with the 2 diff sides its all ya need really, ~$35. i would get the fine/coarse though vs fine/xtra fine or others combos, the fine/coarse will handle all sharpening from very very dull up, if its fine/xtra fine ya are gonna need another stone if the knife is really dull .
norton india stones also work pretty good, i use WD40 on mine, they are also fine on 1 side/coarse on the other ya can get a 8"X2" stone for ~$20, i have had a norton stone for years, other than the diamond they are my fav for free hand ............