As the gameboy didn't really have fog effects like that, a possible solution is to animate the tileset in a way that gives the illusion that the player is underwater. I think you could do that by using solely autotiles for maps like this.
Another solution if you don't want to do that is to change the tone of the graphics to something a bit more darker, that way it feels more like you are travelling underwater. Right now this screen doesn't really say "underwater" to me.