Welcome to the forum first off...
Snappers will feed on fish, but mostly dead or dying. They are not efficient predators of healthy fish. So I wouldn't worry about them cleaning your pond out.
As far as being a danger to swimmers, here is my opinion. Out of the water, snappers are nasty! I mean NASTY! They will snap your finger off in a heart beat. However, if they are in the water they are pretty darn docile. They will generally head the other direction if approached and even pull into their shell if stepped on. I wouldn't stress it.
