I had a good time with this game :)
The gameplay is simple but enjoyable. The fact that the player has an underwater inertia is a good idea: if you run on a vertical slope, the player is pushed upwards and so you have a kind of extra jump. I think this mechanic could have been exploited more.
The puzzles are far too simple but they do not slow down the overall flow of the game.
At first I thought that the octopus would be too talkative and would slow down the flow but it's fine. The narration is simple and effective.I would have liked to see more backstory about the octopus, the world and the shark but I think you lacked time for that. By the way, the final battle is funny^^
About the musics, I saw that you composed 5 musics, good job for that. The musics are very simple but they fit with the overall atmosphere of the game.
On the graphic side, it is a bit poor, more details (little rocks, fishes, seaweed, etc.) would have brought more life. Anyway, you made this game alone within a short period of time and it is still fully enjoyable without this.