

Yeah, also knows as a 4R25 6V battery, or the battery from a “24 hours lamp” (from way back when they still used lightbulbs, they last weeks now on LEDs). Those lanterns are used a lot in marine work and roadworks. Having one land in the sea is entirely common.
I think these have basically always been carbon-zinc, or alkaline.
Technically, zinc is a heavy metal, but basically harmless to humans unless you eat the whole thing. Alkaline batteries contain managous oxide, which is a heavy metal, but you’d need some serious chronic exposure to get into trouble from that.
It’s still a good idea not to open random crap that washes ashore, because there are definitely FAR more dangerous things that come in small metal cylinders. Like unexploded ordnance.