If you really want to hold throw two anchors out the front of the boat. Yes both out the front. I do it all the time. You will need a lot of slack, meaning yes, you have to have more rope out than it is deep. A lot more rope. With two 30lb mushroom anchors and 100 feet of rope on each, you can hold an 18 foot Lund ProV in 30 feet of water on sand with 30 mph winds. With that much rope out your boat will be over the fish and the rope and anchors will be no where near the fish.