The bombs can be either the Seaforium Bombs (taken from the Workshop) or the Huge Seaforium Bombs (from inside the defending keep). This can only be achieved at one's own keep; only bombs set by an opponent can be disarmed, and only in the short period between when they are set and when they explode.
This is a cumulative achievement, and need not be completed in one instance.
Patches and hotfixes
Patch 3.2.0 (04-Aug-2009): Added