Spiral eel grass along with numerous other submerged plants will grow to depths where it receives enough light for photosynthesis. As the plant numbers increase the water usually becomes clearer and clearer unless there are regular sediment disturbancers such as silty runoff, wave action, or animals "working" the sediments. When submerged plants get dense many species can easily grow to depths of 5 to 10ft deep. I had curly leaf pondweed growing in 14 ft of water where I could see the bottom. In clear water ponds, I've seen eel grass growing to heights of 2-3ft in 15ft of water.
