The park was originally scheduled to open in November 2018. Phase 1 is NOT the water park but in fact the Kingdom of the Sun. My source is my Vidanta Grand Luxxe "Celebrate" contract in which I own 3 weeks annually in the Grand Luxxe in Nuevo Vallarta and 5 weeks in even years in the Grand Cascades resort in the theme park. The contract gives a specific date in November of 2018 and even stipulates that if the park does not open by that date Vidanta has breached our contract and we can cancel our ownership and receive every penny of our money back.
I would expect that they will try to avoid the refunds and re-negotiations with all of the "Celebrate" contract holders. There are a lot of us and it will create a lot of work and headaches for them. The water park section is expected to be open in mid 2019 with the Kingdom of the Moon shortly behind it in 2020.
If you look in the arial views the Kingdom of the Sun section is clearly far more advanced than the water park section. The Kingdom of the Moon section is mostly untouched. Even the area with the golf course hasn't been dug up yet.
We took a hard hat tour of the underground tunnels back in October of 2016. It was really cool and massive! I'm sure they are trying for Nov 2018 but it would not surprise me if it runs over. They have to get licensed before they can open and corners won't be cut in that process.