So the 17 acre pond I'm on has pretty dense weeds until you get about 20 ft off shore. I always cut some weeds and drag them out. I'm wondering if anyone has ever thrown this on top of their garden for mulch/compost?

I have no idea, but there may be some considerations for what kind of weed it is, as to whether it will be good for your garden or not.

Yes, we save algae and coontail for the compost pile. If it's been treated with any chemicals, copper, diquat, glyphosate, etc., we don't use it.

Yes I have used it for years and so have a few of my neighbors. It's always a good day when neighbors come over and ask if they can harvest a couple of truck loads for their garden