Children first. That's the most important. From then, form up queues. Anyone who wants to get off sooner can jump into the water and swim for it. Anyone else must line up and get on in an orderly fashion.
Should physically and mentally disabled people and elderly people get to go on first?
No. That is working both against natural selection (Social species, doesn't matter as much, IDC) and forcing misery upon others for little gain.
Now, think about this. Which would hurt you more: Your grandmother died, or your mother did. Odds are, your mother. Your grandmother would be old, and you would know she would die sometime soon anyway. In addition, she probably isn't the main one taking care of you and providing for you.
On the other hand, you expect your mother to be there for many years to come, and she likely takes care of you on a day to day basis and provides a portion of your financial support shared with your father.
If someone must die, let it be the old. Personally, I would try to make it myself the last person to get on. I can swim quite well, and would jump off before a the main part of the ship started sinking so that the drag of it going down wouldn't be as strong. I am intelligent enough to know how to survive in such situations, and have experience in situations similar to this, though not on the open ocean, from my days sailing. I could also likely swim to and hold onto the side of a life boat and help myself survive. On top of that, I don't feel my life is worth that much compared to others, and would gladly save others before myself.
Baring that though, Elderly people will contribute the least to society, are the most expected to die anyway, are the slowest of the lot so it would likely take more time to load them all up than to load on a full set of adults, and are usually not as important to a family as a middle aged parent is.
When you form a queue, there should be nothing to complain about. The order is 100% luck, so the elderly who were up near the front will likely get in before the fit up the back. It is a lottery of who will get on, and thus nobody should complain, and if someone feels so self entitled that they should get to go on first as they are more important than others, I'd throw them off the boat (Not really, but that sort of behaviour annoys me [I should go first! I'm a woman/elderly/disabled/millionaire/ect!])
Really, children first, queue up after that. Sorting out who should go first by physical condition, contribution to society, ect. takes far too long. In that time, more people could be unloaded onto liferats, and if there isn't enough then more people could jump for it and hang onto the sides of the rafts before the ship went under and its pull got even more powerful.