[hyeparis]

When visibility is low, I usually go to the bottom and then look upward - that way you can see siloettes of fish as they pass by above you illuminated by light
but if it is as low as 40 cm, then you have to ask around/read up on what stationary fish are out there in your area and where they hide. they you again dive close to bottom and crawl to the hideway, making sure you produce no noises - such as fins hitting rocks, etc
I sometimes even push myself forward by grabbing rocks at bottom. The more silently you approach the hideway - the more chances the fish won't flee.
If it is deep and water is murky, than obviously visibility will be lower at the bottom than at the surface levers, so you will need a flashlight
However, some fish are attracted to light while others are scared by it - factor in when deciding whether to use a flashlight