They can't stop a team in a small boat from boarding a merchantman, positioning it cross-ways in the canal, then sinking it. Do this once at each end, and your carrier group just lost it's mobility and much of it's ability to project force.
Here's an interesting lesson from WW2 which shows the danger of previously unknown coastal torpedo batteries: https://en.wikipedia.org/wiki/Battle_of_Drøbak_Sound