Explosive decompression would not occur in a submarine at 30 meters below sea level if a small opening, like a bullet hole, were made. Instead, the high external pressure would force water into the submarine at a high velocity. The pressure at this depth is approximately three times that of atmospheric pressure, meaning any breach would result in significant inward force rather than an explosion. While a bullet hole could cause damage, it would not lead to immediate catastrophic failure. The primary concern would be the potential for hull integrity loss, leading to collapse.