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Kudos to the third crew for pure guts getting into such a craft with a proven failure rate; while not an outright suicide mission they must have had considerable presentiments of disaster... and a terrible resolve to carry out such a mission. The situation in a nutshell: Required reading https://en.wikipedia.org/wiki/Underwater_explosion

The overall effect of an underwater explosion depends on depth, the size and nature of the explosive charge, and the presence, composition and distance of reflecting surfaces such as the seabed, surface, thermoclines, etc. This phenomenon has been extensively used in antiship warhead design since an underwater explosion (particularly one underneath a hull) can produce greater damage than an above-surface one of the same explosive size. Initial damage to a target will be caused by the first shockwave; this damage will be amplified by the subsequent physical movement of water and by the repeated secondary shockwaves or bubble pulse. Additionally, charge detonation away from the target can result in damage over a larger hull area.

http://science.howstuffworks.com/explosion-land-water1.htm

One hand grenade is in the pool, the other is by your chair. Which blast is more dangerous? If you chose to jump in the pool rather than endure the explosion on land, we have some bad news for you: You made a disastrous decision. Assuming the blast from both hand grenades was identical in strength and that you were the same distance from either blast, the underwater explosion, also known as an UNDEX, would be far more dangerous. Water, however, is often described as incompressible. Technically, it can compress, but it takes a massive amount of pressure to apply a small amount of compression. This means that in an underwater explosion, the surrounding water doesn't absorb the pressure like air does, but moves with it. An underwater explosion doesn't propel objects through the water nearly as far as a surface explosion throws shrapnel because of the drag water exerts on objects. However, an underwater explosion transmits pressure with greater intensity over a longer distance.