There are 3 gacha: Wiz gacha which gets you items to dress up your little creatures, Whale gacha which gets you items to place on your whale's blowhole, and Pet gacha which can win you little pets for your Wizzes.
Basic gameplay will be leveling up your ammenities which auto produce gold for you. Then you hatch and raise Wiz creatures to auto produce hearts for you. You then use gold and hearts to level them up more. When youre not leveling up and hatching, you can decorate, dress up, feed pets, feed the whale, pet the whale, clean the whale, poke the blowhole for hearts and gold, take pictures of whale and Wizzes, and open clams that randomly appear. Oh and collect ingredients and cook. And probably more I havent doscovered yet (I am only whale level 7).
Missions keep you on track to leveling up and you MUST do them to level up your whale (which is basically like a player level) as this is the only way to get the water droplets. Sadly sometimes these will require you to watch an ad but it is very rare... I have been playing all afternoon into the late evening and I would say the game only forced me to watch 2 or 3 ads total to continue.
If you absolutely hate ads you can buy ad skip tickets for real money. i think it is like $3 to skip the next 50 ads, maybe cheaper overall if you buy more at once.
There are limited costumes you can only get through mtx. And a single wiz costume is like $5... it also comes with gems (premium currency) so youre basically buying a gem pack but still $5 for a costume is steep. But I really want that bee costume... hahaha.
There are also other packs you can buy that contain skip tickets and stuff. And then limited time packs and stuff... as expected from freemium games.
And of course there are offline earnings, and as you level up your whale you will be able to increase the amount of time you can spend offline before being limited.
In the time you spent reading all this you probably could have figured out all this on your own by downloading the game, but if you want to know the opinion of someone 5+ hours in hardly using ads and no mtx, I can say its pretty worth the time sink if you like the aesthetic. Only thing I am worried about is the game will probably slow to a crawl soon, as all these games do, and I will just be logging in now and then to click a few things and quit. At least I feel like I am making progress clicking clams and tending to my Wiz needs (as it increases their heart level, something you can only increase while actively playing).
Anyway just go try out the game. It seems pretty quality for a free idle game.