If this cache is for your workers, then do you need to have it accessible by the general public? Lock it up tighter than a drum in heavy duty containers with hard-core locks that only your workers have keys for. Inside that container keep a maritime radio, a reasonable portable antenna, and solar recharging gear for the battery. All inside a convenient day pack. Include instructions on radio use if your workers are not familiar with that. Also, a map to the top of the nearest high hill if the route to the top is difficult. Sure, you'd have to be able to hike to the top of the hill to use the radio effectively, and you might have to wait a day while dead batteries recharge via the solar, but from up high I'd think you could probably get the 20-30 miles (???) to the mainland or to a passing ship. You might not have a long transmit time with weak/discharged batteries, but you could hopefully at least get an SOS out before they died again after a partial solar recharge.