How Can We Colonize Mars?

I periodically read about people wanting to colonize Mars. Elon Musk seems interested in doing so. However, I'm always confused on how we can actually do that, but not in the technical sense.

According to the 1967 Outer Space Treaty, we're forbidden from sending a mission, robot or human, close to a water source in the fear of contaminating it with life from Earth.

Isn't colonizing a planet basically the definition of "contaminating it with life from Earth"?

I've always thought that it made a lot more sense to create a self-contained spaceship that people would live on indefinitely. They would have to be big enough to hold at least 10,000-40,000 people in able to preserve genetic diversity.

We could send out several of these each in different directions with the idea that only one needs to survive for the human race to continue.

The objective wouldn't be to reach someplace specific, it would just be to continue to live, breed, develop culture, etc. Eventually, there would be a need to create more ships so landing on planets or meteors might be needed, but only for raw materials, not to colonize.

Some people along the way may decide to stay on a planet, but frankly, they would probably die unless it was a whole lot of them.