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If you bought the piece then it may just be as waterproof as it will ever get. Most potters use a glaze before firing to make certain that the ceramics can be used safely and without leaks. If you want to know how the waterproofing is done read on.
The best way to waterproof if you are the one making the figurine is to make certain you fire it to the proper temperature. There are some cases where waterproofing will not take completely. Low-fired clay for example will not hold water over long periods of time. Clay alone and glaze do not bond well enough to provide a completely waterproof seal.
Ceramics are finished using a firing process with temperatures measured by the cone. It is a potter's thermometer to gauge the level of heat in the kiln. Different clay types cure at different heat ranges. For porcelain that would be 2600degrees. It takes that much heat for the glazes used to mature. Cones temperatures are labeled between 06-10 but this is for 100degree increments so do not get confused. The firing process with the glaze is what produces the waterproofing.
Most potters agree that proper firing is the best way but they are also using items like Tung Oil, Thompson's water seal and acrylic floor polish. These are items that you can find in hobby shops or hardware stores. If what you are trying to waterproof will hold food or liquid these choices are not a good fit. Each will give off a toxic residue. This method is best used for flower pots and vases.
If you want to protect a outdoor planter for instance that has beautiful artwork on it you can use a spray on acrylic sealer. Use the water seal on the inside to make it waterproof and the spray on the outside to protect the beauty and luster of the painted surface.
With all types of ceramics it is the glazing during the firing process that truly makes them waterproof. Some of the types of clay compounds do not graze when fired and this causes leaks. Especially when you put a hot liquid in them. The solution is easier if the figurine is not going to be used for anything other than a decoration. But if you plan to say put them in an aquarium you could end up killing your fish. All of the waterproofing products mentioned here are toxic to people and animals.
