You have found yourself at the right place if you want to know how to choose a pond sealant. I have personally heard of people using all sorts of things to seal their pond or water feature with. I have heard of people using asphalt, tar, latex paint, liquid rubber paints, various unknown epoxies, polyester resins, deck sealers and even spray paint, just to name a few.
While there are all types of sealants available to make something waterproof, most of the list of products available generally has to be thrown away simply because they are toxic in nature to fish and plants. If you are building a water feature that will house either of those two life forms, then you need to steer your project towards a sealant that will do the job and not kill your stock. So that is the very first thing you need to consider. Many sealants can actually leech toxins off into the water and cause serious harm. This can happen rapidly or in some cases over a long period of time which could also lead to misdiagnosing the illness being seen in the stock.
The next things you need to consider is what the pond sealant is made of. I keep using the word sealant here because generally that is a recognizable terms when it comes to waterproofing. What you should not confuse the term with is a type of material that is used to saturate a surface and repel water or moisture. These types of sealants are not really sealants per say but repellants that normally need to be reapplied on a regular basis.
What you are looking for is a pond sealant that will not only waterproof, but bond to the surface it is being applied to. Bond is very important because without it, the sealant will eventually fall off of the surface in which case you will see peeling. When peeling happens, water can get behind the sealant and the decomposition of organics in the water can actually expedite this process. Rubbers and latex materials are prone to peeling because their bond strength is inhibited by their flexibility.
You also need to choose a sealant that was specifically designed to work under water. Too many times people take this simple fact for granted. Just because the sealant is capable of waterproofing a surface does not necessarily mean it can withstand the rigors of existing under water. This is also a reason why so many sealants can fail.
Flexibility is also important. It should be considered very carefully because too much or too little can cause premature failure of the sealant. Rubber for example tends to have the highest amount of flexibility, but with that comes its inability to hang on to the surface it has been applied to. Epoxies on the other hand have exceptional bond strengths. This means that when they are applied, they tend to not want to come off. Arguably though, most people tend to think epoxies are a bad choice because they think epoxies are brittle and are prone to cracking through. Generally with the average epoxy this would be correct. However, a good epoxy sealant will not only have great bond strength, but it also has a certain amount of flexibility built into it that allows it to move with the surface applied to and not be prone to peeling because of it.
Finally, the pond sealant should be designed to work properly for a decent amount of time. The last thing you want to be doing is recoating every year. The effort in preparation alone does not even equate to the stress cause on your stock each time you have to empty the water feature, clean it, recoat and fill it again. This does not even take into account the new cycle the water feature needs to go through in order to hold your stock again.