What if enemy fire sank an assault pontoon?

How would the pontoons survive enemy fire, and did they have to remain usable after the landing?

They were treated as expendable after a night landing at high tide, with an escort for protection and boats available for later lifts.

The interpretation offered is that the pontoons were largely expendable. The monitors would approach at high tide during the night, put the troops ashore and withdraw. A surviving pontoon could support another landing on the next high tide, while boats would carry later troops if it had been lost. The force was intended to link up with troops already ashore, and an escort would accompany the assault.