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davidcyclebackI had a bunch of just water damaged 1959 Topps. They looked fine but were warped. When I put them in top loaders so they were flattened out, one wouldn't know they were water damaged.
The number one thing is to remove any wet cards from holders or books and dry them out as possible. Any fungus or such damage is likely permanent, but drying it out will prevent too much more deterioration. If they are older, valuable cards, fungus and other damage will substantially lower the value, but they will still have some value. If a 1951 Bowman has a splotch or two on the back, someone will buy it. There are many collectors of lower grade cards.
Some collectors might find it interesting to own a card that survived the Katrina hurricane of 2005. That's known as interesting provenance, especially when handed down to the grandkids in 60 years.