I've seen this company trying to mass-produce amphibious vehicles lately, that I think would be useful in the Sixth World:

Gibbs Sports Quadski: Quadbike/jetski combo. I can imagine these being used for recreational uses, and by pirates, smugglers, runners, scouts and by marines taking a coastal approach.

Gibbs Amphitrucks: There's the Humdinga (Weird name, I know) that's like an amphibious pick-up, and the Phibia (Makes slightly more sense) which is the equivalent of a van, with all the implied utility.

How would you use them?

The DUKW got an upgrade at last?

... they use a lot of those old DUKWs, or new construction based ont eh same overall design, as Tour vehicles in Boston .... and in Boston Harbor. "Duck Tours", they call 'em.

Modern Day Seattle has a company running a few of those as well.

Seattle Duck Tour Site