Where you think those weeds are growing on ? ---> structure
Why don 't weeds grow like a carpet ? because weeds only grow where they can find the adequate soil substrate ( bottom composition aka structural feature ) & the right ammount of light, those change when structure changes, a hole in a weedbed indicates either a change in bottom composition or a change in depth ( change of the bottom contour, bottom contour = structure ). The shape of the weedbed is telling you on what the weedbed is growing, and it reflects the shape of what 's underneath.
Got topo maps ? those help a lot when they are available, they tell you where everything is.
Got sonar ?
Have you looked around the terrain above the water level ? what you see above water level is most likely going to continue below water level.
What do you know about where certain plants grow ? in my neck of the woods willows only grow where there 's moist soil year round, that only happens in my neck of the woods along the river & creek channels, if I see willow trees it 's only a matter of following the way they are placed on the lake, I know the channel is at their feet or not very far, a creek/river channel is a structural feature.