The armor that the Cloudflow soldiers were wearing underwater were light and properly sealed up. When the mechanical soul was blown up by the grade-8 spell into hundreds of pieces, the pieces melted into metallic fluid that solidified in the cold water to become sharp and lethal shards that pierced through the swordmasters' armor.
The ones whose limbs were injured were still able to use their sword aura to stop the bleeding temporarily before swimming towards the surface. Some of the metal shards shot through the swordmasters' lungs and heart. Blood oozed out of the noses and mouths of the swordmasters as they fell paralyzed to the bottom of the lake. The water seeped through the holes in their armor and they died very quickly.
The mechanical soul's body was not new and its defense was weak. It would have been blown to pieces if not for his unique attributes and high grade.