How to make an object hold water
I've found that a double-walled base seems to give better results than trying to make the base very thick. Any imperfections in the print, or cracking caused by removing the print can lead to leaks - but if there is layer/infill/layer, the result seems to turn out better for me.

Vase mode can be effective (although won't be easy to combine with the above unless you slice your model in several parts), and I've had success whilst using 100% vase mode for vases to hold water. This demonstrates that a single wall thickness is sufficient to achieve a watertight print.