Thanks! Yeah, I'd considered that after I posted. I'm planning to add a gradient layer set to Multiply so it gets lighter towards the top and darker the lower you get. And/or adding more luminescent sealife.
Added the gradient lighting so it's brighter further up. Also added some sea plants, set to a random image out of three I've drawn (all graphics are hand-drawn) with a random colour tint, distributed on a plant map:
The forum shrinks the pictures a bit, so I took a smaller screenshot so it didn't resize so much. Yes, it's still fairly dark, but it's meant to be deep down - there will be brighter areas further up nearer the surface. There are scanlines and noise effects added.
It's turning out pretty well. Thanks for the comments!