It is at least superficially plausible. From this pdf , pages 7 and 9 respectively:
http://www.storysmith.net/Articles/Blast%20Injuries%200406.pdf
“Medium in Which the Explosion Occurs Another important factor that defines blast injury patterns is the medium in which the blast occurs. An underwater blast wave causes far more damage, because water is essentially incompressible.2,16A wave resulting from an underwater blast travels farther and moves faster than a wave from a similar explosion in the air. Blast injuries in water occur at greater distances and may be much more severe.17,18Personnel treading water are at higher risk for abdominal than thoracic injury from an underwater explo-sion. Fully submerged victims are at equal risk of com-bined thoracic and abdominal blast injuries, but the blast injury occurs at 3 times the distance from the underwater explosion”
“Gastrointestinal Damage Gastrointestinal injuries were once thought to occur with the same frequency as lung injury. A recent, large Israeli case series found that abdominal injuries were seen only with massive trauma.33In this series, all of the patients sustained their injuries in open air. The patient may have a greater risk for gastrointestinal injury when exposed to an underwater blast.17The GI injury of primary blast injury is inconsistent in presentation. Gastrointestinal injuries may not be appar-ent externally. They have a great potential to cause death and may be much more difficult to protect against. The injury may consist of hemorrhage beneath the visceral peritoneum or may extend into the mesentery, colon, and cecum.36,37The colon is the most common site of both hem-orrhage and perforation.17,36This is thought to be because the colon has the most bowel gas accumulation in the GI tract”
However, a bit more searching reveals that it is the gas-filled spaces of the body that are most vulnerable to shockwaves when immersed.
From here:
http://www.scuba-doc.com/uwblast.html
"MECHANISM OF BLAST INJURY IN WATER
The blast wave passes through the body as it is of similar consistency to water. Molecules are displaced very little except in gas spaces capable of compression. Damage is at the gas water interfaces within the body. The gas in the gas filled cavities is instantaneously compressed as the pressure wave passes & the walls of the spaces are torn or shredded as in barotrauma. Damage occurs in the lungs, intestines, sinus & ear cavities. In the lungs the damage is not due to pressure transmitted via the upper airways (as in air blasts) but as a result of transmission of the wave directly through the thoracic wall.
ANIMAL EXPERIMENTS
INTESTINAL damage: subserous & submucosal haemorrhage; perforation. No kidney, bladder, liver or gallbladder damage. If the thorax & abdomen were immersed, the lungs would be more affected. If only the abdomen were immersed the intestines were most affected with rectal bleeding.
The above results show the importance of the air-water interface in damage from an underwater blast. If 3 loops of bowel are experimentally occluded, collapsing 1, filling 2 with saline & filling 3 with air, only the air loop is damaged."
So if you’re in the water, an explosion shockwave is more likely to damage your lungs than give you a lethal enema! If you’re half-in, half out, then you can suffer intestinal damage and the consequent rectal bleeding may have spawned the legend of the enema effect. I suspect that covering your buttocks isn’t going to help very much, whatever the case.