Economics is underpinned by scarcity vs the opposite.
Just coming across several trillion tons of water would completely devalue their unit of currency. This find would have been the beginning of serious depression that would have dominoed into the complete collapse of financial quidproquo that cement their society.
The Kazon Ogla would have been idiots to take the Ocampa's water, or admit that it exists, or behaved like they had a secret stash... In fact the most sensible thing they could do to preserve the status quo, and their on apparent wealth, would have been to destroy the underground Ocampan reservoirs.
Which just leaves the magic replicator technology that was threaded through the Ocampan City, of which any one terminal therein was more than capable of remaking that ocean or making a fair stab at the impersonation of any other ocean even if it was the size of the entire universe given enough time and power...
Or were the Kazon too slack witted to forecast their own doom?
If they weren't, then that just means that Caretaker was too slack witted to forecast the Kazon's ability to intelligently side step doom and ruination of their civilization.