Generally speaking tunnels are much safer than outside due to the lack of falling debris. And the fire issue is solved my any half decent sprinkler / ventilation system.
Maybe it’d just be a big thud and the car goes into limp mode. Worth modeling and having risk assessment on either way.
As to the notion that there's lithium metal in the batteries that could react with the water, you're thinking of non-rechargeable lithium batteries. Lithium ion cells use a lithium compound, not just pure elemental lithium. The formulation might be something like LiFePO4 but the point here is that it's not just lithium metal sitting around inside of it.
Also Tesla has an emergency response guide that includes instructions on fighting a battery fire. It's on page 22 (23 of the PDF) and it makes it clear that you want to douse the thing in as much water as possible and keep dousing it until it's totally gone. Tons of water is exactly what's called for when fighting these kind of fires.
https://www.tesla.com/sites/default/files/downloads/2016_Mod...