It was assumed that there are few fungal relationships with aquatic plant roots.
The reason is simple: fungi are aerobic, the sediment is not.
There are a number of fungi that have been found, but their roles are likely somewhat minor.
I found some ectomychorrhizae on Bolbitus.
Other ferns likely have it.
As far as roots, there's bound to be some, but they are not as useful for aquatic species as they are in terrestrial systems.