Question: Why did they have to use a bomb to blow out the door to slow the Hermes for the rendezvous with Watney? Wouldn't simply opening the door have the same effect?
Answer: From Internet sources: The crew blew the airlock hatch to change course and rendezvous with Mark Watney after he launched himself from Mars. This was done to overcome a critical situation where the crew lacked sufficient fuel to travel to Watney and the be able to return home. The blasting air venting into space would propel the ship forward while saving fuel.
From my take of the movie, they blew the hatch to slow the Hermes to more closely match Watney's speed, not propel it forward.
Yes, they blew the front airlock, so the thrust would be aimed against their travelling direction. It may be confusing if you are not used to the idea of 'braking' with counter-thrust.
Question: What are the potential problems that Watney talks about in his video log?
Answer: The harsh environment of Mars. The batteries had limited power. The food could only last a few months not years, same thing with the living quarters and equipment.
Question: Would it really be possible to make water using the method Mark Watney uses in the movie?
Question: The gravity acceleration on Mars is 3.71/ms2 a little more than double our Moon and approximately 1/3 of Earth gravity. For this reason every falling object or walking/running people or bouncing object should be attracted to Mars in a "slow motion" fashion. Watching the movie I didn't notice such slow-motion effect. Was that a deliberate error in order to avoid huge additional costs and probably making the movie less tedious?
Answer: Most likely, yes. Though it is worth noting that is probably also part of the reason why the number of falling objects on screen is kept to a minimum, and there is one more thing: there is much less air resistance on Mars than there is on earth because of the thin atmosphere. For a real world comparison, look at the footage of Apollo 15 where David Scott drops a hammer and a feather, and the hammer doesn't fall much slower than it would on earth.
Answer: Airlocks have two doors - open door 1, get in, close door 1, change atmosphere inside airlock, open door 2, get out. It's a built-in safety feature that only one door at a time can be open. As they couldn't overcome it, they had to blow out one of the doors while the other was open.
With all the geniuses on board, they couldn't figure out how to jumper a safety interlock on the outer door? They figured out how to bypass several redundant computers to change their trajectory to rendezvous with Watney.