You should be afraid. You can't accurately predict the situation whereby the
fender will be a hazard; therefore, the best approach is to eliminate the
hazard,

Based on this idea, I should also keep everyone tethered as well and wearing
PFDs with signal lights.
Really, Jonathan! You're being very silly. We're talking about two fenders on
deck in calm conditions for a few hours. NO WAY can anyone be hurt by them
unless they explode and create a time vortex bringing a T-Rex on deck and
thereby capsizing the boat.
Stop being silly.

RB