Cover over gas tank, should it be soft?

[Parrot6909]

Well, here is my first in what is sure to be a ton of ?'s for you guys. Lets introduce the boat, 1986 V20. The floors and transom are solid, but there is a very noticable flex in the area above the gas tank, 1/4 to 1/2" of sponginess.......Was it designed that way or is it time to rip it out and redo it? Just an FYI for size, I am 6'3" and about 225. Not hopping on it, just walking over it....Help is appreciated!