Good luck Victor! I would also recommend a good underwater metal detector or at least one that can be submerged up to just before the control box. I know that there are some VLF machines that will work in water and highly mineralized soils but a PI machine would be the best option. These would be great for the deep cracks and crevices of the bedrock and larger rocks on the bottom of which you cannot see what is in them. The great thing about a PI machine is that most of them are great in highly mineralized soils but you would still be digging every signal as they do not have discrimination. I hope to get to some great Gold areas in North Carolina this year and put my' PI machines to work in the streams, their' banks and the adjacent areas below the flood line. There is one such stream that has drainage from an area where an 8 pound Gold Nugget was found in the 1880's and usually if there is a Momma Bear, then there are sometimes some little ones and occasionally an old crusty Grandpa, LOL!!
Frank