Soft spots in the deck. Some of the V21s had a bench seat fastened to the deck in the rear. For that reason some may have spongy deck in the rear. Also, the forward rod holders on my 94 drained directly into the foam under the deck. I fixed it by running some tubing from the bottom of all 4 rod holders back to the bilge. The best way to see if that foam is soaked is take out the rod boxes, but the foam being soaked could rot out the stringers, bulkheads, deck, etc.
It seems like quite a few of the V21 ended up having soft floor issues. The above may be the reason.
If the owner kept it covered and/or garaged, these were probably mitigated, otherwise if left outside or in the water then I'd be suspicious.