It would be more expensive, but a gasoline / air mixture might be more effective.
Napalm would have been good too.
Yeah, but you’re not going to be able to get any evidence afterwards.
Additionally, there is video of Hamas tunnels containing goosenecks and similar gas traps. However, none of it looks to be particularly proof against multiple kilotons of water.... and if it is, the water just traps the occupants inside on an ongoing basis in a way that a fuel air explosion wouldn’t. Plus you don’t accidentally blow up any of your troops conducting operations that didn’t get the word.