I tried to write for clarification, and couldn't even ask my question clearly! (Humans communicating in person >>> typing on the internet.)
Jadog, based upon your response it appears the following two statements are correct:
1.) The entire area of the "beach" is currently dry. Therefore, now is by far the easiest time to perform any necessary construction.
2.) A significant proportion of the "beach" will be underwater when the pond is at "normal" full pool water level. The deepest part of the beach will be at least 5-6' underwater.
If those two statements are correct, then you will get significant beach sand losses in a 5-acre pond due to wind erosion. Basically, the waves will put the sand into suspension, and then gravity will move it down the slope of your underwater shore line. That is what needs to be remedied by any of your preventative measures.
You say you have a pier close to the beach. If people might dive off the end of the pier, then 5' water is too shallow, regardless of any berm or barrier you may build that makes it more dangerous.
If you post a little more info, then some of the experts can probably give you some valuable advice.
