We recently added beach or mortar sand to our pond where we created a small beach. We’ve found that where the sand is in the water is is very soft like quick sand and you sink right down. Will this harden up or should we add something to the area where sand will be in water?

it'll depend on wave action and the sand depth - you might try to add some "play sand" or coarse sand to mix in to help solidify the mixture. Usually very fine sands are more prone to becoming "quick sand" from what I've seen in rivers and streams around my neck of the woods.