I think the effect is good but the placement would be wrong for Disneyland's version. My understanding is that the guest is traveling through time. The caverns are in the present where we see what's left of the pirates and their treasure. Then you pass through a dark tunnel and emerge finding yourself in the past and right in the middle of a battle. The ride up the waterfall at the end is your portal back to the present. A mermaid skeleton in the caverns might make you wonder. Did mermaids actually exist at one time? But a live mermaid appearing in the caverns would throw off the original time travel concept. If it were to be done at DL, perhaps just outside the cave near the battle would be a better place for a mermaid sighting.