The narrator already summarized the episode pretty well: "Conducting a rescue and being rescued in turn". And I would like to add it was a really funny and curious episode. So, let's get started with the details:
Most importantly, of course, there was Lapras' return. Frankly, it has never been one of my favorite Pokemon because during the Orange Archipelago adventures you couldn't help thinking Lapras just served as some replacement for a boat for Ash. But still, it had it special moments, and the reunion today was emotional, too. So, yes, I guess it was a good idea to bring it back.
Then we had that constant back and forth with Wailmer being rescued, evolving, being stuck again, being rescued once again and finally Ash being the one who needed help. I wasn't quite happy with the first two rescue attempts because I'm not really sure if pulling such a heavy Pokemon out by just using a simple fishing rod or a man-powered small submarine would actually work. However, I really loved the idea behind the final rescue of Ash: Freezing the incoming water and digging a tunnel into it. That sure was cleverly done!
for me the two funniest moments were a) Pikachu enslaving TRio which, as some other user put it, was slightly unethical
and b) Wobbuffet giving TRio energy drinks randomly and out of nowhere. Both scenes were hilarious and made me laugh hard.
Finally, my favorite scene clearly was Misty's final remark "We just could have used a Pokeball to free it". YES, of course you could!!
While you could have wondered why nobody had come up with that idea earlier, it kind of shows to me the writers actually start thinking about their own episodes, which is a good think per se. However, I'm also wondering why they didn't do that during the PWC and the Masters Eight Tournament in particular.
Anyways, an entertaining episode that was really fun to watch!