Ok, 3HD. Looks like Str is pretty decent (could stick to 14-15 for the +2 to damage, but I think it would be fair to bump in the conversion). And a 1d6 bite. Nothing else to speak of, I guess.
That's what I was thinking.
Might as well go for Str 15.
20 ft. crawling and 50-60 ft. swimming.
Downsize a Purple Worm, change it to an Aquatic Vermin, and give it Str 15?
Purple Worm: Str 35, Dex 6, Con 25, Int 1, Wis 8, Cha 8
Medium Worm: Str 11, Dex 12, Con 13, Int 1, Wis 8, Cha 8
Hmph. Better cut the Charisma too:
Amiskwia: Str 15, Dex 12, Con 13, Int --, Wis 8, Cha 2, NA +2 ?