You may only need it once, or maybe never...but it might save your life. Replace it somewhere.
As Dale H said, one cubic foot probably isn't going to kill you, but all of the foam under your seats being removed..... is not a good thing.
You will only know when you sink the boat.
richg99
p.s. One time, I foolishly moored my boat, floating, partially under the roof of my buddy's boat lift. Texas torrential rains came that night, and tons of water spilled off of the roof, and into my 20 ft. Gulf Coast. The bilge pump gave out sometime during the night, and my boat was gunnel to gunnel filled with water.
I had to step onto the boat to get a jumper onto the bilge pump. I don't which ..."last cubic foot of flotation"...kept her from sinking at the dock...but I was glad I had every last inch of foam that day.