One of the interesting differences between monecious hydrilla (ours) and dioecious (southern US) is that the monecious may produce seeds, dioecious does not. Spread though is almost always related to fragmentation.  It can be spread by wildlife if part of the plant is hanging on them when they fly to a new location, but the rate of spread is as compared to being moved by humans much, much lower. The hope with all invasive species is that if we can slow the spread enough, and control populations long enough, natural completion can develop- predators, diseases, etc.
 
As far as fishing beds of hydrilla, at first it appears that fishing improves as the fish get pushed to the outside edge making it a great target. However as the bed expands, the plants grow so thick that not many fish/amphibians can survive within. I actually found a sunfish last summer that somehow got on top of a mat of hydrilla and was unable to get back down into the water. I managed to knock it back into the lake with a reach pole from my boat.