<p>I made a tea bowl, but it leaked when making it in PLA...</p>

<p>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? </p>

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.
Some tips:
Printing in vase mode can save loads of time, otherwise the printer wastes time doing each layer separately.
Your nozzle should be 0.4 mm and up, otherwise the print will take forever.