Parachutes use a "3 ring" setup to release the main chute in an emergency. It's basically 3 progressively smaller metal rings mounted on webbing which are interlocked with each other and held together with a single pin. As such, there are no real "moving parts" so it should be very reliable as long as the rings are made out of stainless steel, and the webbing is waterproof. There's a picture at
http://www.altico.com/universal.html (not a very good one) The release pin goes through the light coloured cord at the top of the smallest ring. In this arrangement the pin carries no load, so should always release easily. It could be safetied with thin nylon or cotton which would need a definite pull to break it, thus minimising accidental release