Funnily enough, the guidance for extinguishing lithium ion battery fires is to drown them. Most consumer battery fires are due to shorting through damage. A good case in point is the increase in small fires on aircraft from phones and tablets. trapping a dropped phone between the seat squab and back and crushing/bending it when changing the recline angle has occurred often enough that airlines may include this as a briefing item during the safety brief people never listen to.
"A small Li-ion fire can be handled like any other combustible fire. For best result use a foam extinguisher, CO2, ABC dry chemical, powdered graphite, copper powder or soda (sodium carbonate). If the fire occurs in an airplane cabin, the FAA instructs flight attendants to use water or soda pop. Water-based products are most readily available and are appropriate since Li-ion contains very little lithium metal that reacts with water. Water also cools the adjacent area and prevents the fire from spreading. Research laboratories and factories also use water to extinguish Li-ion battery fires."
The Dutch fire service has used a large container that can be filled with water at an accident scene to apply this approach to whole vehicles.
Yes, lithium metal can react with water to produce hydrogen, which then burns. But you tend to have to work quite hard at it.
Generally speaking, though, that's not salt water. Of course, lead-acid batteries have their own problems if they contact sea water, arguably more dangerous than a little hydrogen fire.