Welcome to the Bunnings Workshop community @Ryinth. It's fantastic to have you join us and many thanks for your question.
There are probably a few different ways to make a pot watertight. A method that has worked well for me in the past was to seal the hole with silicone. I started by cutting two 5cm x 5cm pieces of plastic out of a milk container. I then applied a generous amount of Selleys 300g Wet Area Waterproof Silicone Sealant - Clear to one of the pieces of plastic and stuck it over the hole on the underside of the pot. Once it had dried I flipped the pot over and filled up the hole on the inside with silicone making sure to contact all sides of the hole. As a final safety measure, I cut another piece of plastic, applied silicone and stuck it over the hole inside the pot.
Alternatively, you could silicone the pot down to a saucer. Just apply a liberal bead of silicone around the pot base and place it in the saucer. Once that has dried you could run another bead of silicone around the join to make sure it doesn't leak. I'd still recommend filling up the drainage hole with silicone just to be safe.
We look forward to seeing your water feature in action and remember to post some images when you are done. I trust our members will be interested in seeing your results.
Please let me know if you need further assistance or had any questions.