What is it about fishing stuff that it often has cutting edge circuitry? These cute little flashing lures are no exception with literally a regulated 5V supply inside them that runs from a single AA cell, either disposable or rechargeable.
The units use a mini boost converter to power a standard flashing LED, and will run at full intensity down to about 0.8V and then get progressively dimmer down to 0.25V.
Even with a relatively low capacity NiMH AA cell they have a run time of many hours.
They have two seals. An O-ring and a secondary tapered compression ring to make them extremely waterproof.
Note that the barbs are an optional version and they are just glued in. I managed to pull them out while they were cased, so maybe not great for monster squid.
I could see other uses for these cheap lights as decorative tree lights for adding sparkle to trees in outdoor events without any need for wiring.
As with many items these days, these came from AliExpress (not a sponsor):-
https://www.aliexpress.com/item/1005007046856981.html
Cost is typically less than £2 space credits.
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https://www.youtube.com/watch?v=zRi1Ec5Ddu0