When I think obstacle course, there is a possibility of a mud crawl. So, water resistant doesn't cut it.
Based on the info supplied in your post (undefined "abuse"), I see two paths:
1) don't risk the FB just to track one day (its not your typical step day) or
2) pay the bucks and get a waterproof case (like the cell phone cases)
For me, I would never spend the bucks to get the case for a one time event
Best alternative advice: insert the FB in a sealable zip lock bag and insert the bag into pants/short pocket that zips up. Won't prevent breaking, but provides limited water protection.
| Body Fat %: 17.0