The main game starts moderately difficult, but then gets easy fast. It wasn't very long before I was immune to every hazard, and so big that all I could do was sweep up and down the screen eating tens of fish in a matter of seconds. The boss battle was a pretty big spike in difficulty relative to the game, giving the difficulty curve of this game a V shape. Not something you usually see. I'm an absolute sucker for games like these so I enjoyed it regardless, but there are some things you should consider if you ever make another one of these.
-Make the evolution more interesting. The evolutions in this game were just gaining immunities to things. The last evolution adds something interesting with giving you the ability to eat with your entire body. Changes like this that freshen up the mechanics at a steady pace are what makes games like this so compelling.
-The playing space was really cramped. If you made it bigger there would be room for larger enemies, and as such it wouldn't be as easy to become so hulkingly huge that it just becomes a waiting game.
-If you're going to make an upgrade screen, put a lot of different upgrades in. Speed, agility, and evolve aren't enough. It would have been really cool to be able to upgrade hazard resistance and your shield and such individually instead of grouping it all under Evolve.