Kids are (or at least were) used to such darkness with fairy tales like the original version of The Little Mermaid where the mermaid loses the prince and dissolves into sea foam, The Snow Child about a child made of snow who comes to life and then melts when placed in front of a fireplace and don't get me started on The Gingerbread Man lol
Having said that, I don't think the water clone was necessarily an anti-marriage plot point as it was clearly a cliffhanger setup for a larger story about Peter having to find the real MJ. Unfortunately the show ended before their plans could pan out.