Land Battleship and other giant land vehiclesThe main two issues here are mass and size.
- Mass: Any wehicle that big will, quite simply, sink into the ground it stands on - and if that ground happens to be muddy or swampy, good luck trying to get it out. That is, if it will even move at all - the engine required to move something this huge would probably take up most of the vehicle, move it slowly, and, unless its world has some exotic power source, consume truly horrifying amounts of fuel.
- Size: Imagine taking a megatank through a mountain pass. It'd be a horror - such a vehicle would be too wide to fit on a road, and too heavy to climb the inclinenote . And you don't even need a mountain. The mighty vehicle of mass destruction would get stuck trying to climb a hill the moment it leaves the plains most such machines tend to dwell on.
Submersible carrierLeaving aside the difficulty the plane would have returning to its mothership, and the problem of waterproofing the airplanes (and their jet engines), an aircraft carrier is the exact opposite of hydrodynamic, meaning it would move slower and require greater amount of power to push it through the water than its non-aircraft carrying colleagues, necessitating larger power plants. Larger power plants mean far greater noise and heat generation, which increases the sub's visibility - and the entire point of making something submersible is to make it as close to invisible as you can.
- That being said, Imperial Japan during World War II did build submersible aircraft carriers- the largest non-nuclear submarines ever built, in fact- the I-400 class submarines, which can carry three Aichi M6A special-purpose floatplanes capable of carrying a single Type 91 aerial torpedo or up to 850kg of bombs. These were intended to destroy the locks of the Panama Canal, but the subs surrendered on their way there in 1945 due to Japan having done the same back home. However, the aforementioned drawbacks are still present- these things were huge note for a sub and can only carry three aircraft.
Amphibious TanksYou're in luck! Those are actually real.
Amphibious AirplanesHere the problem is threefold - speed, mass and propulsion.
- Speed: The chief enemy of a plane attempting to go underwater is speed. At speeds high enough for the plane's wings to lift it, meeting with the surface of the sea would be roughly equivalent to colliding with a concrete wall, and while the plane's aerodynamic shape could carry the main body into the water, the wings would be essentially torn off.
- Mass: Planes are generally built to be light - otherwise, they wouldn't be able to lift off the ground. Unfortunately, that also means they are not particularly inclined to go down (luckily), so if you managed to get your plane underwater, it would pop back to the surface due to being too light to sink (assuming the plane is waterproof, that is). And if it didn't, the light mass doesn't make the plane particularly tough - the water pressure alone would crush it at even shallow depths.
- Propulsion: To put it shortly, jet engines don't work underwater, and neither do aircraft propellers.
That being said, amphibious planes as in "planes that can land or take off from water" are real, and there are two types:
- The first type are known as flying boats which have a hull that allows them to land on water but usually precludes them attempting to land on dry ground, though some of them do have landing gear to land on ground; examples of this are the Imperial Japanese H6K, American PBY Catalina, or British Short Sunderland.
- The second type is a floatplane, basically a plane with floats to land on water. The German Arado Ar 196 and American OS2U Kingfisher are examples of floatplanes.