The boat anchor is conveniently called a para-anchor.
They are used most typically used in very heavy weather such as a hurricane and are used to slow the boat to a controllQuoteable speed by producing drag through the water (wind on the surface will push a boat much faster than the current is moving). Most para-anchors are built specifically for this and are of to porous a material for what I assume you would be using it for not to mention that one the size you are looking for would cost approximately $800.00.
I personally am using for my para-anchor an 8' cargo parachute that I picked up at an Army Navy store for twenty bucks.
