The Casio W-800H is fairly resistant to varying environments, displays dual time, and is also lightweight with a 10-year battery life. Oh, and it's not terribly expensive.
The W-800 is a wee bit larger compared to the base models, and it has a world/dual time and 100 meters of water resistance, more than enough for snorkeling. This model doesn't allow setting a countdown timer by default, but there are instructions out there to mod the circuit board in a very simple fashion to activate it. At your own risk, of course.
Features
These are features that may require clarification and may not be found on all models:
This model has no automation and allows you to set a secondary time down to the minutes.
Stopwatch with 1/100th precision up to 24 hours. Displays elapsed time and can be toggled for split times and 1st and 2nd places.
The split time in Casio watches means freezing the elapsed time on the screen while the stopwatch keeps running in the background. E.G. if you run 1:36 laps with a steady pace, your first split time will be 1:36, the second one 3:12, then 4:48, and so on.
You can set hourly time signals on or off, and the watch has a multitude of alarms: daily alarm, daily alarm for one month, alarm at a specified date, and a monthly alarm. You can set the chosen alarm to snooze if you like.
You don't have to check the date setting of this watch each month: leap years, Februaries, and Biblical events are covered for a very long time – automagically.
This watch has one green LED for viewing in the dark but it's better than usual: there's a light guide panel to distribute it more evenly across the display. The Afterglow is fixed at 1.5 seconds. Higher-end models of today have something they call the Super Illuminati but this one is just fine.