I made a tea bowl, but it leaked when making it in PLA...
What are the key points to keep in mind when designing and printing an object that is intended to hold water using an FDM printer?
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.
When designing the object, you should make sure your object is completely enclosed (obviously). When printing, try increasing the print temperature so that the layers stick to each other well. The most important thing is the print temperature, because if the layers don't adhere to each other well, you will get a leak. The wall thickness of the object can be thin, as long as it isn't so thin that it has too little strength to hold anything.