No; not right now, either. It's a lot loser to 1/12; Earth's average ocean depth is ~3700m.

That may have been shallower at some point in the very distant past (which, to be fair, also describes the last time Mars had surface water), but for as long as Earth has had continents it's probably been within rounding error of that value.

OK, so *some* of us breathe lake water instead

...jeeze

Doesn't Mars have a lot more than than 300m elevation variance on its surface? How is 300m of water supposed to "cover" it?

It doesn't, though. At least, it doesn't for entropic and quantum definitions of "information", which are the only ones that matter in this context.

For perspective on what that all *means*:

If you were to travel to the Centauri system (~4 ly away) at a velocity of 0.85C, the trip would take just over 4.7 years. Because of the time dilation (which is actually 1.7, not quite 2), it would seem from your perspective that the trip took a bit over 2.76 years.

In order to preserve the speed of light in your own local reference frame, it would also appear from your perspective that the spacetime you passed by was compressed by the exact same amount as the time dilation factor (so it would seem like you had only traversed ~2.35 ly in that 2.75 years, giving a velocity of 0.85C).

At the speed of light, it doesn't. Seriously, time ceases to pass for anything travelling at C.

For anything moving through spacetime, the passage of time is slowed. The magnitude of this slowing (the "time dilation factor") increases as velocity as a fraction of C increases. Yes; this means that right now you are technically experiencing Special-Relativistic time dilation because of Earth's motion through spacetime (and have been for your entire life, so it's all you know).

This is not a linear relationship; the time dilation factor is very small until you achieve a significant fraction of C. It doesn't reach 2 (so time is passing at 1/2 the base rate) until something like 0.85C. As your velocity approaches C, the time dilation factor tends to infinity (and the passage of time slows to an arbitrarily slow rate).

Technically speaking, nothing with mass can travel through spacetime at C, only at arbitrarily-high fractions of C (which also takes inconceivable quantities of energy, but that's a separate discussion entirely). The key words there are "through spacetime", though. If you can find a way to bypass spacetime or modify it in some way so that you can cover a longer distance faster without actually going faster *through* that spacetime, then you can avoid the Special Relativistic effects. That includes the time dilation *and* the prohibitive energy costs.

Yup; we *only* need to find something we aren't certain can even exist.

That mostly depends on whether or not matter with negative mass can exist in this universe. If it can, then the Alcubierre drive is possible (once the insane power requirements are resolved). If not, then he best that we know is definitely possible is around 0.1C. That gives us a transit time of 10 years per light-year, or around 40 years to the Centauri system. That's with Project Orion (a rocket-like vessel powered by literal fusion explosions).
So unless we can significantly increase the fraction of C it would probably be literally a once-in-a-lifetime thing, if that, for most people.

There are some theoretical possibilities, like replacing the fusion explosions with antimatter explosions (or even some sort of actual antimatter rocket) or the Alcubierre drive. Both of them require currently-unattainable quantities of special materials.

Any sort of antimatter-powered vessel capable of carrying people is going to have a mass measured in thousands of kg (probably many of them) and will require facilities for either storage or in-situ production of similar mass of antimatter for a given trip. Somewhere around 0.2C is where the payload:fuel mass ratio hits 1:1 (assuming in-situ production isn't an option, and of course making assumptions about the maximum efficiency of the engine).
We can currently only make antimatter as individual subatomic particles in particle accelerators, and our current facilities aren't designed for that. With current tech, a facility optimized for it could supposedly make about 20 grams/year. That means 1 tonne (1000 kg) would be 50,000 facility-years of antimatter production with today's technology. I think it is highly likely that this will change in the future and we will actually be able to produce enough to use as an exotic power source for special applications. I suspect that the energy cost to produce it will always remain prohibitive compared to other manufactured fuels, but I'm also sure that at some point our energy production capacity will (eventually) trivialize that by brute force.

As for the Alcubierre drive (which would theoretically allow travel faster than C), it's dependent on a hypothetical type of exotic matter that has negative mass. It isn't used as a fuel, but to generate the negative gravity well on one side of the "warp" field. We literally don't know if such matter can even exist in this universe. It's mathematically predicted by some symmetries, but that doesn't necessarily mean it actually exists. That could simply mean that something else is missing from our models. At any rate, even with the Alcubierre drive generating the field, there still needs to be some kind of energy source to empower the drive, and it takes a lot of power. We're talking planetary masses' worth of fuel even for hypothetical antimatter reactors (and I mean gas giants).