The problem with Ash is that he has little to no stakes in some of the movies. He's only there because he happened to stumble upon a legendary or town with connection to legendaries. It's the same problem I have with the main anime when it comes to people around Ash having more reason to have emotional and physical investment. Why isn't the person with a direct connection to the movie plot the protagonist and Ash the supporting character? This kid literally pushed May out of the climax of her movie.
This is all the more reason it's a shame they didn't make any Sun and Moon movies, where Ash isn't just a drifter passing by big events by chance and there's a chance to make things more character driven on his end.
For what it's worth I liked Ash setting up the temple thing in that movie, it shown his protectiveness of May well, but I feel like after that May should have taken stage for the final climax, sort of like the Power of One where Ash is the hero but everyone's helping him get to that point, Temple of the Sea could have had May the hero but with her big brother surrogate's heroism pushing her to make that big stand.