Are there any myths about a leafy or weedy sea dragon?

Answer 1

Yes, there are, though this tale is a of a lake dragon and not a sea one. The most famous tale is probably that of the Drac which is tale from France. According to the legend, the Drac in appearance was great in girth but its hide was as the lake bottom, all reedy and covered with green growth. In the most famous tale, the Drac lured a young woman from a local village by floating a goblet on the surface where she was washing clothes. When she leaned forward the Drac seized her, pulling her down to its watery realm and there it had her act as a wet nurse for its own offspring. One of her jobs was to rub a salve into the infant dragon's eye, thereby giving it dragon sight, however, she would often absentmindedly rub her own eyes after applying it. When the young dragon was weaned, the Drac returned the woman to the surface since it did not see it fit to devour her who had nursed its young. She resumed her life, but due to the dragon sight she perceived the Drac one day in the market square, prowling for unattended children. She alone could see it due to the dragon sight and she called out at it. The Drac covered her left eye, then her right, and determining that the sight was only in the left, the Drac struck out her eye.

When the Midgard serpent arose from the depths to come and do battle with Thor at the battle of Ragnarok it was said to be covered with all manners of sea life: seaweed, barnacles, clams, remoras, etc.