Gameplay concept is very nice, making a creature out of functional constructor parts and fighting other creatures with it is quite entertaining. However, you could make it even more interesting if you included an element of exploration and puzzle-solving - having your levels a bit more complex in design and require some usage of parts to progress further (like these breaking stones from tutorial and water streams that require more speed to swim against, they force you to adapt to a task). Perhaps this could be another game mode, I'm sure you already thought of something like this if you included it in tutorial (though, at the very basic level of course). An example of this kind of tasks would be a task to push a spike debris into the big enemy's mouth to make it swim away and clear the path.
Creature parts and their method of obtaining are another topic: while some of parts are functional and very rewarding to get, there are some parts (like this new energy leech) that simply aren't as good as the rest are. As a result, because bosses drop random parts a player can get a part that is useless or nearly useless to them or just doesn't fit their play style. For a player that just defeated a boss in hope to get something they wanted, this is a let down. Even bigger let down when they reach final level and realize they will not get any better parts that they missed. Partially this will be solved as you tweak the parts to balance them out to make them all beneficial (like, maybe add a tiny bit of instant damage to the mines so that a large cloud of mines would actually destroy something on impact?). Another way to solve this problem would be to give a player a choice to go for a certain parts and work for it instead of railroading them to a boss and giving random reward. An example would be some kind of hub area where player would decide "okay, I want to get pusher part so I'm gonna dive into specific whirlpool that leads to the sea level" they dive into that whirlpool, go through several sea stages, fight the sea creature boss, get some sea creature part, perhaps pusher, then they return to the hub and choose another level. Some new whirlpools appear in the hub area, leading to next levels, but player can choose to dive towards previous levels again if they didn't get the part they wanted. This makes a non-linear way to play through the game as player will have a choice between first few levels and will be able to perfect their creature the way they see it. That hub area could also eventually lead to puzzle levels and endless survival levels where player would fight increasingly stronger opponents (inspired by current final stage). These solutions would improve gameplay freedom greatly and I guess they don't require too much of a change since you already have whirlpool mechanic in place.
Graphics. The art is awesome, it looks intriguing and unexpectedly detailed for an indie game. Staging the game setting in the sea was surely a right decision as it gives player a lot to look at; darker themes of later levels successfully set up a more serious and creepy atmosphere. It makes me want to explore and see more of it. Especially the gray blank walls of nodev facility (I'm not serious, I know it is missing texture at the moment). But yeah, I sure want to see more levels and creatures made by this artist. Perhaps even something H.R.Giger-style. Also, maybe you guys could post some concept art from the game development etc.
Sounds and music are done well enough to complement the gameplay and overall mood, can't say it's extraordinary, but nothing to criticize either. Good job whoever did it.