Hi Davo,
I thought about sending my water loving dogs up to pull the weeds out for you as they love doing that in my dam. Seriously now, are the water weeds a normally occuring plant or are they a noxious type. If they are a natural type and not choking the dam I would leave them there as they are a part of the natural cycle and provide a food and oxygen source for water creautures. If it is a noxious weed, contact the DPI or local council and see what they would recommend. I wouldn't use round-up as it is highly toxic to fish, frogs, etc. One non-chemical method would be to get a long rope and have one person on one side holding one end of the rope and one on the other side holding the other end and drag the rope through the water to pull the weeds out. This may not get rid of it totally but used regularly may help keep it under control. Be careful you don't fall in! Use the weeds in the compost or as a mulch - great plant food! Hope this is some help. Ps. I like the idea of doing the tropical gardens. We are doing the same thing, so far only in the backyard (it's big) but will be extending it to a gully in the paddock and eventually thoughout a couple of acres. We have about 5 acres all up but have to keep some as a paddock for the kid's ponies. We started most of our plants by cuttings and by tubestock and now our poinciana trees are nearly as tall as the house and as wide and are providing shade for understory plants. Cheers | 
