Good concrete, well cured needs nothing as a sealer. Many on here have used an admixture called Xypex which will, supposedly, create a crystal structure through the concrete blocking any pores that water could find its way through. Be sure that your plumbing is pond plumbing and not swimming pool plumbing as there is a significant difference in the power of the pumps, and the amount of debris to remove from the pond. The only coating that I would use in a pond is the same one used by the swimming pool construction, which is well tried and true to hold water. Many of the paint on have a high frequency of failures, pealing off. Concrete really does not tolerate coatings very well. Most are film forming which allows vapor pressure to build up in the concrete and put them off.
Pool contractors doing shotcrete tend to leave the shotcrete too rough for the finish surface on a pond. Get them to go the extra mile and give the concrete a finish more like a floor.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard