Lauren,
what an engaging prompt, although, as you have remarked in response to another teacher, an actual physical scenario can be striking too. I would expect the physical letter, (slightly water stained?) would be very effective.
The scenario you proposed would lend itself to testing materials that hold or surround the mermaids tail when it is submerse. Did you make the mermaid tail out of card or sugar paper? If so, it would be very easy to see if the tail had become wet when submersed.
I found this craft of making a mermaid from a paper towel roll: https://www.thecrafttrain.com/cardboard-tube-mermaids/
There could be an interesting discussion with an older class on the breakdown of plastic in the ocean. See this adult level article: https://www.kqed.org/science/1941258/after-3-years-in-the-ocean-these-supposedly-biodegradable-bags-are-still-intact