1. Due to Kni Engine, the game is actually running at 1920 x 1080. Due to how fullscreen works, it forces the game to run in 1920 x 1080, which actually shows off screen elements (i.e bubbles spawning off screen)
2. Idk what exactly you mean by player acceleration making movement feel unresponsive. The slow fall is due to you being in water (and holding down is meant to be used to dodge more effectively) and horizontally, there isn't any acceleration at all. Could you clarify what you mean?