I was thinking the Swim check to escape once already swallowed, but it could probably be expanded to the initial attack.
Sure, let's bump the HD a bit. An orca is Huge, with 9 HD. These start at about the same size, so use the same HD?
Orca ability scores: Str 27, Dex 15, Con 21, Int 2, Wis 14, Cha 6
Spot-checking a few fish, they've got Int 1, Wis 12, Cha 2.
So maybe Str 27, Dex 15, Con 21, Int 1, Wis 12, Cha 2? Possibly drop Str by 2?