As many others said, I think seeing threats only when in the player's light area makes the game way too hard :(
However, I really like the concept, bullet hells being rare during jams (I think) and fitting the theme perfectly here, the music being really good and stressful (which is kinda coherent with being hunted by a giant fish 300 meters under water). And the fish's design is really good too !
If you post an easier version after the jam, I'd love to play it again and finally see the end !