If the undine is especially fishy looking, you could also try the Reaper "goggler" (fish-man) that carries a net.
But I agree with Sean, probably easier to find the mini that looks right in terms of the rest of appearance and then glue some net-looking cloth to the hand. If you have a hobby store near you, you might find some netting that model ship builders use (though I didn't have any luck finding any online, but I'm sure I've seen stuff like that before). You can also soak the cloth in some diluted PVA glue so, once dried, it will stiffen a bit if you don't want it to flop around on the mini.