If I have seen correctly, amphibious vehicles like the LVT series have very rusty tracks in service use, presumably because they have gone through the sea. Can anyone provide further information?

They rust extremely quickly. Even when operating most of the track remains mostly rusty. Only those parts that are getting "rubbed" by the guides, sprockets or wheels and in direct contact with terrain have the rust temporarily scrubbed to shinny bare metal. Then shortly thereafter those areas rust and everything begins again.
On AAV7s the EAAK used to rust quickly too. On the RAM/RS the roadwheels rust quickly as well. The UGWS also show rust after a while in service.