Various sorts of commercial fishermen use these to keep the buoys of their traps submerged until pickup time. This prevents both accidental loss from fouling boats and poaching.

You can get them in grades from 1 to 30 days, though temperature also gets to play a role.

http://www.neptunemarineproducts.com/galvanic-timed-releases...

Looks like you can also use them to prevent "ghost fishing" where a lost fish trap continues to trap fish for years or for timed bait release schemes.