The long/tall vegetation that grows in swamps is as much as a hindrance as the water. The blood may drop to the ground below the long stems of the swamp grass, be down low on it, or on the underside. One must part the grass with their hands and look. One method I use is to look for water splashed up on the vegetation. Walking deer don't splash water and badly wounded deer splash a lot. Many times this splashed water will be tinted slightly red. Water splashed high up on vegetation meas a wildly running deer. Tall swamp vegetation is hard to see thru when you are on the ground, but it is noisy. Trail quietly and stop and listen often. Wounded deer will many times head for deeper water.