With settling Mars we mitigate a number of potential extinction events, and fulfill some science fiction dreams. Living in the ocean also helps with both goals, but to a much lesser degree.
Say you wanted to create a city sized operation/colony. If we start by comparing to vessels and platforms already in operation we see that the price tag for such can vary between $250 million and >$1 billion for both cruise ships and oil platforms. Those costs are for established technologies with existing production infrastructure and with the assumption of constant resupply and a market for the goods and services they produce. If you were to break ground on a completely new area the costs would be much, much higher.
There's also the question of what special benefit we gain from moving an operation to the ocean when compared to doing something similar on land. Both are still on Earth with all its pros and cons but the ocean simply multiplies the engineering challenges. You could ask the same question of space but the difference is that a space based facility has several major advantages that go hand in hand with the challenges it presents. Namely, greatly reduced gravity, vast amounts of 'rare' elements, the potential to move large quantities of material with a comparatively small vehicle/engine, and the prestige that comes with a new frontier.
Finally, doing one does not necessarily mean we neglect the other. There are many areas where advances in one could benefit the other and there are proposals for systems that would see a great deal of overlap between the two, such as the Sea Dragon.[0]
A terraformed Mars makes sense, but it's not exactly impending. If you just think about it terms of colonies on Earth, colonies existed because they had something to offer the "mainland". Sugar, rum, slaves, spices... something. Colonies without that benefit didn't stay colonies for long, and what's the point of creating... I don't know... SpaceHaiti2000?
Cleaning the atmosphere and terraforming those deserts is probably much easier than terraforming Mars.
I have enough faith in our race that even if we have someone who is genetically less fit for mars we won't just let him die before he reaches a reproductive age.
So will it be by their potential partners? It would be pretty funny if they end up developing martian dating apps that requires putting their DNA sequences online just like how height and race is currently required.
This isn't how selection pressure works. And while you're correct that we won't let a human die, potential partners already do a sort of DNA analysis. While it's not as ruthless as in the wild, potential partners definitely evaluate each other based on criteria like fitness, appearance, and intelligence, which have genetic components.
Consider that the Martian environment might make individuals who are genetically more susceptible to skin cancer likely to have facial scarring before reproductive age, or make lanky individuals more graceful in the low gravity, or make short people able to move through small hab units with less awkwardness. Or consider the potential effects of claustrophobia or a proclivity for motion sickness on life in space, or the demands on intelligence and resulting careers.
These effects would cause some genotypes to be more attractive than others in a different way than they are on Earth where, for example, height is often considered attractive and is not penalized by short space hab ceilings. It's not that those lacking these traits would be prohibited from reproducing, but that they would have a bit of a challenge finding partners, while those possessing the traits would find it slightly easier. As a result of a few on the margin failing or succeeding in finding partners, those with the traits might have 2.55 children on average, and those without might have 2.53 children. Eventually, this pressure could shift the gene pool in a different direction than Earth.
To have sex, yes, but I'm not sure it translates in any meaningful in the number of surviving offsprings we have, therefore eliminating selection pressure at the human scale. We have removed a good chunk of selection pressure, and there's nothing wrong with that.
Selection pressure, however, is still a very real phenomenon for the micro-biology inhabiting and interacting with our bodies.
Hmm, as long as the separate species are still physically compatible *cough, I don't see why that would be necessary, at least in the context of dating/hookups.
For breeding? Sure.
Not that there's anything wrong with that. :)