Strickman, I have used the Sandshark for years to sweep bedrock underwater where I dredge--for me, it's only been sensitive on nuggets around a gram and up--but I really don't like leaving gold of that size behind. As far as trying to find gold smaller than that underwater, I would rather keep on dredging as the dredge season is very short, and I'd rather run yards than chase specks. I guess it's what you really prioritize for underwater detecting.
I've used it on land in red hot ground that eats VLF's for lunch, and found good gold too, but in the same size range as mentioned above. As it's a PI machine, it has its limitations--and I spent a lot of time trying to find a more sensitive coil for smaller gold that would fit it. I contacted them (Tesoro) directly and asked them about it and I never got any promising responses. (Hung out on the forums for a long time asking questions too.) Maybe they have one now for it--in all fairness to the manufacturer--I haven't checked recently. But, it's a great little machine for underwater work, or work above ground in horribly hot soil, and it's quite simple to operate.
P.S. If anyone knows of a sensitive little elliptical that will function with the Sandshark, let me know please.
All the best,
Lanny