I'd agree with esshup about the slope, but since you would have to build this peninsula in up to 16 ft of water, I wonder if the soil wouldn't tend to slide out even farther. You would just end up with whatever slope the natural repose of the submerged dirt would hold.
I've been fortunate to build peninsulas before we built the dams, so we were able to properly shape them, then protect them with rip rap from wave damage.
