You have to pump it over the seawalls and barriers to start, but I’m also assuming that Hamas might have read everyone’s field manuals and is including grenade sumps, mantraps and other features that might keep low pressure water out. Pump it in at high pressure and watch for geysers to figure out where all the entrances were. Makes it easier to track down supporters that might have fled and give them ‘special’ attention.
Blow the sea wall up.
Just flood the whole damn place.