This speaks of the need to have more than one signaling device, based on the environment. At sea, flares could work...if they work (lots of misfires in them). How many can you carry and shoot off? Signal mirrors are great but only on sunny days. A dye marker in the water could be really effective IF they fly close enough to see the trail. With today's technology I think PLBs are the way to go, land or sea.
Gedi Convertible Helmet
Canoe / Kayak Anchors