Copied from the original letter this was written for…
Real Ship(s): “The Caspian Sea Monster”
The lesson here is “Just because the engineering is cool, doesn’t make it useful”. The “Ekranoplan” or “Caspian Sea Monster” was a (now long abandoned) attempt by Russia to exploit something called the ‘Wing In Ground” effect. Basically, as a plane flies closer to the ground, the air pressure below the wing builds up. Kind of the opposite of how a normal plane makes itself get sucked up into the air by fast moving air causing low pressure above the wing during takeoff.
In theory this made it fly below radar detection, which would be good. In practice it could only do so above water and despite all the math working as expected, the prototype was tested for 15 years before it was abandoned after a crash.
Specs are impressive…cruising speed of 250+, top operation speed of 300+, maximum speed achieved 400+…all while within 30ish feet of the water surface (and sometimes inches above). The wingspan was 123 feet and length was 300 feet. While technically an aircraft, it was classified ( and christened with champagne) as a water vehicle and assigned to the Russian Navy. It was the largest and heaviest “aircraft” in the world until the late 80’s – the plane that surpassed it is the one that you may have seen pictures of transporting a space shuttle that’s been mounted on top of it.
So…it’s a plane, but also a boat, but also it flies, but only on water, and the math is really cool, but yeah it broke so…? You will be shocked to learn that no one else has tried to pick up where this one left off. 
__________________________________________________________________________________
Start Wars Ship(s): V-4X-D Ski Speeder
This one, again, isn’t a ‘ship’. But the pilots we see using it are naval pilots in Star Wars, and Star Wars doesn’t really exist anyway, so cut me some slack…I think the comparison works. : )
At the end of ‘The Last Jedi’ we see the rebels holed up in a base on Crait trying to make a last stand before Luke (of all people) comes to save them. The new order brings out a mega laser, and the rebels try to explode it with what looks to be broken pod racers with training wheels. Enter the Ski Speeder.
I think the real reason we see these is to highlight the visuals – the white surface (salt) being churned up red (iron) looks super cool, but the ships themselves are kind of ridiculous. The V4 ski speeders were developed for ‘asteroid slalom’ racing…which I assume is like sci-fi pilots going ‘mudding’. The rebels ended up buying them on super sale after a bunch of people got themselves eaten by an Exogorth (giant sentient space worm) while using them.
The salt had all but destroyed what was left of them by the time they are seen, which says something about the original materials. Luke’s x-wing is in flight ready condition after spending decades under water on Ossus, so they had the tech to make the vehicles stable, but they were clearly built more because someone thought it was cool than because a mono-ski vehicle that can become a land speeder if it goes fast enough was actually needed or a good idea.
Specs on these are less impressive. 20 feet long, 33 feet wide, 2 lasers, 1 pilot, and 1 mono-ski. Described by Poe Dameron as ‘a B-wing that couldn’t fly’ which is hilarious if you know anything about b-wings, which I’ll try to arrange for if I can find a good comparison.