I'm not saying that the US wouldn't develop such a vehicle, I'm saying the Amtrac itself would have been developed in the CSA as the specific environment it was designed for is in Florida. any US attempt at such a vehicle is stymied by the fact that the marshy environs that lend themselves to such engineering and design philosophies really don't exist in the North. as such, the engineering and development would take much longer.
And Considering the CSA and the British both conducted a Joint operation in the Bahamas and Bermuda, CSa seaborne assault operations aren't entirely out of the question either.
In the same vein, Snow-environment vehicles like the M29 Weasel wouldn't be as common in the South, as the snowy and mountainous environs it was designed for aren't as commonplace in the CSA.
The Amtrac itself will not exist in 191, same as the vast majority of weapons post 1880 as the conditions which created it would be butterflied away. The CSA is unlikely to develop such a vehicle as the Confederate Marine Corps will be starved for funds given the need to have an Army that can fight the US Army
It would take the North slightly longer to develop such a vehicle from scratch, but not all that much longer, probably two years, about as long as the Japanese took to develop theirs from scratch, rather than somewhat under a year
I said opposed landings, there is a difference, in an unopposed landing you don't have to worry and can just use lighters and take your sweet time. The Bahamas are basically going to be unopposed, or nearly so, anyone trying to defend them in that strategic position should be sacked. Bermuda is more difficult, but can be explained as the Brit's doing the heavy lifting, which they would in any case as the CSA has no navy so the operation depends exclusively on the Royal Navy to happen. Again the CSA is not going to be worried about opposed amphibious operations, that is not important to their war
The Weasel was mostly used in tropical environments (literally during WWII and its postwar combat career only 15 of thousands were used in cold weather environment), assuming the CSA could afford vehicle like them they would love them