Gary is the aerospace engineer. I hope he can answer your landing questions in detail. I can tell you that from a layman's point of view we will have many opportunities to test landing methods on Mars through robotic probes and we will test our ability to land large craft before sending any humans to Mars in both mission plans if I recall correctly. Mars Direct doesn't even launch humans until the fuel manufacturing facility indicates fuel manufacture complete. You questions may have somethingt to do with why NASA changed the Mars Direct plan into a semi-direct plan, but I never undeerstood how they thought they were making the plan safer other than increasing crew size to include an MD. Leaving a return vehicle in orbit seems riskier to me than keeping it close for inspection and repair if needed.

How many people do you know interested in colonizing the ocean compared to colonizing Mars? People colonize regions they are interested in colonizing not those they aren't. I think some of the reasons may be people's instictive discomfort with being under water or in lightless areas. The presence of sharks may also have something to do with it. The oceans certainly provide no stepping stone to colonizing the stars which is what most people interested in colonizing Mars see it as. The oceans provide no two basket protection. The oceans provide no opportunity for widescale planetary ecological engineering without risking unintended consequences for Earth. Colonizing the oceans provides no opportunity to develope the technology needed to tap the almost limitless resources in space. In short I don't see the oceans providing the opportunity for major leaps in human capability that space colonization does while being nearly as difficult or even more so in terms of mere human survival.

Regards,
Mark