It might actually survive, I'm pretty sure I swam with one as a kid and nothing happened, would hate to kill it though so might still consider leaving it in my pack.
UPDATE: Well, it's friday now and I'm glad to say it's still kicking without an issue. Crazy it can survive the swimming in the ocean and being submerged for such a low price and WR rating!
Watch enthusiast currently living in Caracas, Venezuela.
If you don't go too deep, it might survive. I think the regular Casios are rated at 50m depth, so that's not great. Someone will correct me on this if I'm wrong. You would be better off with the square G-Shock with a similar-looking case that is rated at 200m depth. That one would do it, no problem.
A cumulative effect of some number of repeat exposures is needed to kill these things. Usually takes over a season.
If you have a diver or a g shock I take that but he f91w will be fine.