> how are you going to stop someone from scuttling a ship? sink it?

Yes. A carrier group is able to project military power for a great distance around itself. Long range missiles, submarines, aircraft, you name it. Presumably it would monitor the canal along its entire length, and destroy anything that approaches by land or sea before it is able to create or become an obstacle.

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.

Admittedly there are edge cases and military planners like nothing more than dreaming these up and devising countermeasures. The Suez and Panama Canals are not small, so it's not as if all the ships have to crowd together and be trapped in a small area. But yes, there are threats and their probability of success is never 100%.

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

Then Egypt has declared war on the United States and is going to have much bigger issues to deal with.

If the US sent in it's fleet when Egypt says no, that's the US declaring war through the act of invading a foreign country.

If you sink it anywhere which isn't directly in front of the canal then the scuttling is a failure, right? A typical anti-ship missile has some 150 km range, seems pretty easy to sink that ship long before it reaches the canal.