it is well known that this element is insoluble in water?
Broadly speaking there are no things that are insoluble, to some extent everything dissolves in everything. Just for some combinations of substances and solvents the saturated solutions have concentrations so low we can - for all practical purposes - ignore them.
Even pure water as a solvent is surprisingly aggressive and capable of attacking almost everything (which is one of the reasons why it is so difficult to produce and store). Sea water additionally contains plenty of ions, which will act as complexing agents. Chlorides are quite good ligands, they will further increase solubility of many metals, gold included.
Gold is sea water typically listed in 3 (or 4) ppt range (3×10-12
kg/L, or 3×10-9
). That's way too low for any economically viable method of reclaiming it, still, with the volume of oceans estimated at over 1018
mass of dissolved gold can be estimated as more than 30 millions metric tons.
Estimates say we have so far mined a bit below 200 thousand tonnes of gold.