Take aquarium safe silicone and white spirit, take a jar put in silicone and pour in white spirit start 1 part silicone 1 part spirit and start stirring, takes a few minutes but it'l mix.. Keep adding spirit or silicone till you have the viscosity you like.. It should be thin enough so you can apply it with a brush.. Now coat the inside of the clay pot with a a few layers of this.. Let it cure for 24 hours and it's water tight.. For the rest treat it like you builded a new aquarium, fill it with water and keep it filled for a week and water change every day to rinse out all silicone chemicals.
Works like a treat, did a few clay pots myself like this.. As long as the silicone layer inside isn't damaged it never leaks water..