Present your plan!

[bridgediver]

That does sound like an interesting approach. My concern here would be at 30', how can the diver/tender be assured they're covering the proper area? If the diver is off by 1' - 2', the weapon could be missed.

Quite true. The tender's skill is important as is practice with the procedure. He would essentially landmark the diver off of references on the far shore while being very specific about where he stands and moves as the pivot point.
We have been successful in training evolutions picking up revolver sized objects routinely using this method - admittedly maybe not with the degree of weeds that you seem to be indicating which is why I'd add a couple of buoys at the max distance out as a sort of boundary that the tender can also use to landmark where he drops the diver down (ie - 3rd pass he'll be about 7.5ft left of the buoy, 4th he'll be 10ft etc).

Not as accurate as the sweeps but maybe faster and simplier than the standard jackstay IMO. Moving a jackstay u/w is far less accurate whether its in the weeds or not as the diver controls the pattern and won't know coverage areas without the surface reference from the tender. The diver can't tell if theres bends in the line etc
-- at least in my experience

Hope thats a little clearer

 

[BladesRobinson]

...Blades, dredging with a drag line might just be the way to go here. The plants do pretty much make it impossible to search effectively. Herbicide might also be a solution, but would it damage the object you're searching for?

Reef,

The herbicide would not have an affect on the ballistic evidence and I doubt it would adversely impact any other evidence. It is a risk/benefit analysis in my mind. Would it be better to retrieve the weapon and potentially loose fingerprints but have the ability to resolve several other pieces of the puzzle ... as opposed to not finding the weapon at all? That is a question the lead investigator would have to answer, possibly with input of the technicians. I am not certain what evidence, if any, would be lost because of using herbicide in the lake. I do know that the water district managers should be consulted, as well as the manufacturer of the herbicide to determine what precautions divers should take as it relates to contamination. My opinion is dry suits, with attached vulcanized rubber hoods & gloves along with full face masks would be adequate under most circumstances (we dive in herbicide laced canals all the time in South Florida) but there is not harm consulting with manufacturer and determine concentration, half life of the chemicals and other risks.

Blades