oh come on krusty...
You seem to think I'm talking about the time of the trip being the issue. No.
LVTs were shallow water vehicles. They could float. That does NOT mean they could sail the seas. There's a reason certain vessels will work in lakes but you should never take them into the sea. There's a reason they loaded these things from ships rather than having them make their own journey. They could NOT survive. Even mild chop will overturn a lot of these so-called amphibious vehicles. They were strictly limited to where they could be used, and did not set out on their own across vast distances like PT boats did.
Your analogy of the plane flying to 30k doesn't work because... frankly... the plane COULD fly at 30k. LVTs couldn't run in deep water. No more than a flat-bottom boat could, I'd wager.