It's a matter of how the structure was built to withstand certain forces. Kind of like an egg - it can withstand tremendous pressure if you use your hand to try and crush it with both hands from the top and bottom, yet you can easily crush it from the sides. The exterior of
Galactica is likely designed to take forces like that from the
outside. However, the charges that took out the water tank were effectively "tamped" - the effects of the explosives were amplified by where the charges were set, as well as the pressure waves created in the water when they were detonated.
There's also the whole "it's in the script" clause that you've got to keep in mind, as well.