Certainly not - it was sea water getting in that caused the huge Krakatoa explosion. See here:
http://www.drgeorgepc...cano1883Krakatoa.html
and read especially this bit:
A Phreatomagmatic Event
Krakatoa's paroxysmal explosion was "phreatomagmatic" . Ocean water entered the magmatic chambers of the volcano when its walls begun to rupture. Super heated steam built tremendous pressure which, in turn, resulted in the large explosion of the volcano. The violent explosion of released gases blasted huge quantities of ash, cinders, pumice, bombs, and larger blocks skyward. The explosion was followed by the collapse of the volcanic remnants into the empty magmatic chambers, thus forming a submerged caldera.