What is a dead battery cell, what causes this to happen, and what are the implications?
In my boat, I have had a couple of lead-acid batteries fail with a shorted cell.  In one case, the shorting may have been caused by the impact of a minor collision, but the batteries were 6 years old, so reaching end-of-life anyway. The shorted cell reduced the battery voltage by 2 volts, and apparently had enough internal resistance to prevent the battery from providing enough power to start the engine.

In both cases, the boat had two batteries in parallel, so the battery with the shorted cell prevented the good battery from being fully charged.  These were all "flooded" batteries, and after trying to recharge the batteries for some time, the good cells in the bad battery all needed water as they were being over-charged, while the shorted cell did not need any water, nor did the good battery.

A battery can develope a shorted cell because the cell plates shed debris over time, and this debris falls into cavities at the bottom of the cell.  When the cavity is full, the debris will contact the bottom edge of the cell plates, shorting the cell.