If you look at our seas, the closest appendices looking like limbs have been developed by cephalopods and starfishes.

They are nowhere close to the complexity of human fingers, but at least in the case of the octopus they allow for quite some degree of manipulation.

Considering that fingers have evolved after a long series of modification from some fish fins and you are excluding this path, I think it's more plausible if you move along the tentacle path.

If you look at our seas, the closest appendices looking like limbs have been developed by cephalopods, starfishes and crustaceans.

They are nowhere close to the complexity of human fingers, but at least in the case of the octopus they allow for quite some degree of manipulation, followed by crustaceans.

Considering that fingers have evolved after a long series of modification from some fish fins and you are excluding this path, I think it's more plausible if you move along the tentacle path or the crustacean path.