Nice, I haven't seen that one! For a moment I thought it was going to be the vehicles they turned into watercraft, ala Jeremy with his Hilux ute and boat engines, puttering down the lake.
It's a fun show, but I'm skeptical of some of their builds. That hovercraft they built out of a van, in another episode, did not appear to behave like a hovercraft at all. And I can't imagine how the Reliant-Robin rocket could have flown even if it had separated from the booster rocket.
Oh, it's a fun show for sure, and I am certain there's some creative licence attached to embellish some (if not most/all) of their creations but, it's the mad-cap nature of it I love, I'll take my science and physics lessons from elsewhere! Now you've reminded me to rewatch the other Reliant Robin clip