Neither seawater or dirt sound like they address the anode or cathode, which seem to be pretty major challenges when it comes to rechargeable batteries.
BTW, speaking of seawater and dirt forming a battery, I recently needed a way to measure underground corrosion of buried copper pipe, and it went down a great little rabbit hole uncovering documents such as this [0]. It was fun to go put a voltmeter across various ground-coupled conductors in my home to see what, if any, voltages were present, with the earth serving as part of a battery.
BTW, speaking of seawater and dirt forming a battery, I recently needed a way to measure underground corrosion of buried copper pipe, and it went down a great little rabbit hole uncovering documents such as this [0]. It was fun to go put a voltmeter across various ground-coupled conductors in my home to see what, if any, voltages were present, with the earth serving as part of a battery.
[0] https://archive.org/details/CAT78694105