Back in 1996 I attended a lorry fire on the M5 in Somerset, UK. The cargo included out of service lithium batteries from (the then GKN) Westland Helicopters in Yeovil.
Waste cotton cloth and vegetable oil had spontaneously combusted, leading to some fireworks as the batteries joined in.
The situation was considerably enlivened when the doughty Fire Brigade turned on the the water taps, and the resultant FireMk2 melted the lorry chassis, and a goodly area of concrete underlying the tarmac.
The County's Scientific Services report was that Lithium batteries (in those days) did contain some metallic lithium, and adding water to the burning cargo encouraged the by-product of hydrogen gas by pyrolysis.
I believe that battery chemistry has advanced and water as a fine spray is now considered a suitable extinguishant for modern lithium batteries, though they do burn persistently and at high temperatures.
I'll see what my ex-colleagues' view is on submerging a lithium battery in water.