The best option for sealing your
pond may depend on several things. One of which is the type of soil your pond is made of. If it's rocky than bentonite isn't likely to seal it particularly well. The other factor is your resources.
So, if your soil is up for it and you have the money to purchase the bentonite I think it's a great option. I'm a bit biased though, coming from the largest producing bentonite state in the US.
Other options for sealing your pond may be pigs, which do work fairly well in my experience but I wouldn't want to put any of my body parts in
water that a pig gleyed.
A pond liner may be a good option if your soil is unfit for gleying or bentonite. Cement isn't something I would suggest as I've been there, done that, and it was awful.