The increased potential for drowning a full harness would present would be the difficulty of getting out of it quickly while submerged - possibly requiring the removal of the lifevest to accomplish. This would be a problem if the hook were snagged. Also, a full harness presents the same "danger" a lifevest can - it provides something for things like tiller extensions to poke through and trap a sailor under the boat. A recent story involved an Olympic sailor having the tiller extension go through arm holes in a lifevest during a dinghy capsize, trapping the sailor until the vest could be removed. Some sailors are wary of vests and the like as dangerous. Others find ways to cover up over top of vests and harnesses to help prevent this.