To study submerged plants in the field without damaging the plants, you can best use an aquascope.
The author designed an aquascope to work in water with moderate to high turbidity. It goes down to 2 meter and proved to work very effectively.
For anyone who wants to construct such a aquascope: you are welcome to download the design.
Easy to buy is the conical aquascope mady by Plastimo: seach with Google
This aquascope is very useful in shallow or clear water and is commonly used, not only by scientist, but also by anyone to view submerged life in warmer areas. It is popular among anglers to view 'their' carps swimming.
(also available in other colors)
You best cover the complete tube with black duct tape to improve your vision; the coloured light passing through is annoying.