Apparently, they are inadequately sealed.
From August 2023:
Floridians battered by Hurricane Idalia this week may not have expected another threat — that floodwaters could cause their cars to suddenly burst into flames.
Yet that's exactly what happened when two electric vehicles caught fire after being submerged in saltwater churned up by the storm. Firefighters in Palm Harbor, Florida, cited the incidents, both of which involved Teslas, in warning owners that their rechargeable car batteries could combust if exposed to saltwater.
Fires can ignite weeks after flooding.
Electric vehicles exposed to saltwater can burst into flames.
Oh.
“Apparently, they are inadequately sealed.”
The government bought a bunch of submerged Teslas and tore them apart. Water intrusion into the battery compartment was due to failed seal due to design and installation problems,