Those amphibious oyster-harvesting trucks that I saw in Brittany are probably not purpose-built but are likely to be modified LARCs - that is, "Lighter, Amphibious, Resupply Cargo" originally built for the US forces in the fifties and sixties. Correspondent D Powell writes:

"They came in three sizes: LARC-V (5 ton, 1 engine) LARX-XV (15 ton, 2 engines) and LARC-LX (60 ton, 4 engines). The French fishermen's trucks [are] either the 5 or 15 ton models. There were also at least two of these (probably 5-tonners) operating 'Duck Tours' type services on Jersey, these were the only passenger vehicles requiring certification under the island's P30 Oversize Vehicle regulations."

Thanks, D! (any advance on D?). Don't forget that those tonnage ratings refer to payload, not overall weights.

It turns out that Searles Sea Tours also runs a LARC-XV known as 'Wiley the Wash Monster' on trips out of Hunstanton.

At the top of this post you can see a smaller LARC-V being unloaded from its very big brother, the LARC-LX:

The LARC-LX was also known as the BARC (Barge Amphibious Resupply Cargo) - read more about it at this excellent site http://www.amphibiousvehicle.net/amphi/B/barcspecial/barc.html all about amphibious vehicles. Below you can see it in full seagoing mode:

The LARC-LX/BARC really is huge: 19m long and 8.1m wide, with a loadspace area of 4.2 x 12.5m. It weighs 92 tonnes dry, but then it has a payload of 60 tons - and an 'emergency' payload of 100 tons!) Surprisingly, it can manage  12mph even fully laden - but then, it does have four 310hp engines.

If you want to buy a BARC (and let's face it, who wouldn't want to?) there are a few for sale, including a couple here, at a reasonable-sounding £50,000, and this one, at a less attractive $345,000 - oh, and it's in Louisiana. Still, there's a lot of metal in a BARC, and you wouldn't let a little thing like the Atlantic get in your way, would you?

You could also visit the sensibly-named BuyADuck, which specialises in amphibious trucks of all types.