To be fair, i think i can explain how it all can work contingent on having 1 specific technology that is within the potential grasp of contemporary humans to get, and it all nicely fits if you think it trough further.
Theory: the key about these flying devices is that they have a propulsion powered by a nuclear fusion power unit.
So lets look at it deeper.
1) Is it possible that some rival to the US has managed a breakthrough in nuclear fusion, but choose to not tell the world about it? Yeah sure, nuclear fusion is often seen as something that is just around the corner for humans to invent, and if Russia would invent nuclear fusion it would have reason to keep it secret given how much their economy floats on the sales of hydrocarbons.
2) could a nuclear fusion power plant explain the form of propulsion and effects associated with this craft. Well yes actually:
-first of all a nuclear fusion power plant would allow the craft to achieve a far greater thrust to weight ratio than typically associated with flying vehicles that are based on burning hydrocarbons, therefore achieving far greater speeds and acceleration. With hydrocarbons you often have this issue that the fuel itself adds mass to the the whole so that there is no linear correlation between between added payload and size/mass of the vehicle, its not because you want to shoot double the payload into space with a rocket that it simply requires a rocket twice as big, no it will be rather something like 4 times as big. So given this potentially far better thrust to power ratio such a fusion power craft would be able to achieve far higher speeds, as it hardly needs to consider the mass of its fuel.
-Second point is that for lack of using hydrocarbons for propulsion the craft indeed would not have a fuel burning jet engine, but rather would make use of electromechanical powered propellers. As such, it could for example propel itself by thrusting out a dense jet of air behind it, but since you're working with the mass of air that is before you another alternative is to proppel the craft by creating a vacuum in front of it.
3) the characteristics of a electromechanical propeller based vacuum drive would be that it can do the things these things seem to do, even in relation to going underwater which a jet engine would not achieve. Propellors will work underwater too, and again a principle of proppeling by creation of vacuum before it would allow to go into the sea splash less as the matter is removed and likely compressed rather than pushed aside. it's somewhat similar to the principle of a jet ski, but reversed by using vacuum before it rather than a jet behind it. There would also be a method to avoid cavitation with this, providing you can just displace the mass before it right behind it once surrounded by water and while youre moving. Picture atleast cavitation as something like an air bubble collapsing on itself trough pressure, inward pressure at that, what you rather might get in this case is that the vehicle ejects pressurised mass behind it that exapnds but in a direction to push the vehicle into the vacuum rather than push directionally against the mass behind it, and thus thereby expanding into the space left behind by the vehicle. So rather than that cavitation would occur behind the vehicle, it might actually occur in front of the vehicle as the vacuum should act as a self imploding bubble in relation to the pressurized mass around it, providing atleast it has the time to do that while the vehicle pulls in mass at a high speed as to maintain the vacuum before it.
4) if you think about what the best shape would be to design such a craft that uses a nuclear fusion power source with electromechanical propellers that needs to be aerodynamic in both air and underwater, you would likely choose .. a tic-tac shape. What is the shape of a rocket that aerodynamically needs to move to space? if it wern't for the jet engines, it would look like a tic tac. What is the typical form of a submarine vessel? Yes, it's a tic tac. No wings needed when one has such a thrust to weight ratio it can perfectly fly around on either vectorial thrust or in fact vacuum creation.
5) You might not be even able to see that the craft is powered by propellors, simply by the rediculous magnitude of revolutions achieved by these with such a power source behind it. These propellors would be a little marvel of science on their own i would immagine, requiring a very high precision with very narrow tolerances. That the craft would apparently be very hot inside though is hardly surprising, fusion might achieve temperatures also produced by the sun, although just aswell friction could heat up such a body however when proppulsion is vacuum based that friction could be avoided, in fact a perfect vacuum has actually a typical temperature of 0°C, so that would explain the coldness on the outside, its as such even a rather clever method to cool the whole thing down albeit that for the temperature to be rather cold on the outside it should have a shell build out of a material that is very low in thermal conductivity like say plexiglass but that can then also explain some sort of "shapelessness". Another things is that a fusion power plant likely would need some water, albeit super compressed even when water is difficult to compress just because you have the power to do it. It would atleast also give potential reason as to why you'd like to take a dive with this thing from time to time, even if h2o could also be filtered out of air.
conclusion: it appears to me thus, that much of the characteristics of those objects could be explained by the simple consideration of it potentially having a nuclear fusion based power source, besides even that if you wanted to design a aerial craft that utilized a nuclear fusion power source that likely it would result in something that akes very much to these tic-tac objects.
disclaimer: i have a basic grasp of fysics, thermo and fluido mechanics aswell, but when coming to such things much is a matter of speculation, i'm just speculating based on what i know, perhaps someone even more knowledgable might correct me on a few things i wouldn't be surprised about it and i'm open to be corrected for function of understanding. ;)