This game employs the so called "metroidvania" style of gameplay. That means you are free to roam around, but full exploration is only possible through finding upgrades that will allow you to get through certain obstacles that you couldn't before. The world is immense and there's a lot of backtracking to be done but the gorgeous graphics and different themed locations, not to mention the masterfully crafted soundtrack, do a great job at minimizing the tediousness. The upgrades themselves are pretty cool, though their usefulness vary and some feel underwhelming compared to others, even though the same can be said about the parent games of this genre. The controls are very responsive and all of the character's moves can be done with just the mouse if the player so desires. The combat is very energetic and being underwater allows for a ton of dodging, which you will have to become proficient at to get through this game since the screen can get quite busy with projectiles and enemies at times and your character can't take a lot of beating. This difficulty can be severely mitigated by the ability to cook many kinds of food, as several of them heal your character, increase her defense capabilities, or even both depending on the kind of food itself. There is a plethora of secrets to find (some useful like cooking recipes and pets that will fight for you, some just aesthetic like costumes), the rarest of them often protected by optional bosses, all of which will very well push the playtime past 15 hours if pursued at a normal pace.
Overall it's an excellent game in my opinion; It hits the right notes in all technical aspects, has an interesting story and a very satisfying amount of exploration and content. Greatly recommended!