I used an ACR Firefly many times to indicate my location to helicopters doing SAR in southern Arizona "a while ago." It worked like a charm every time, rugged, and waterproof. There are probably more advanced, lighter, etc. models out there now, but don't get anything that is any less dim or less rugged. Our standard signalling devices were mirrors by day, ACRs by night.

Later when sea kayaking in the Channel Islands, I bought ACRs for my life vest. The batteries are still good. ACRs are truly good bits of gear. Ahh...memories.......