CasioF108WH Water Resistant Digital Strap Watch

Q: Does this watch have a countdown timer to set to (for example) 8 hours, so it can keep being reset for 8 hours more?

The F108 offers a stopwatch not a countdown timer.

Q: Would like to use as a stopwatch when swimming laps. Can the stopwatch button be used in water?

Don't listen to Mick! He doesnt know what he is talking about. Mick probably will not pull the tag off of him mattress because it is a crime. You will be swimming laps for 30mins to an hour couple times a week. Swim those laps and press away. The watch will likely hold up. If it fails after 6 months to a year, probably not the end of the world, it was $12 bucks. Live a little