Say a passenger jet lands in the ocean. Could the captain being the last person to get off the jet use the drop down dry chemical oxygen generators to breathe while checking for passengers? I know they are flimsier so depending on depth they would collapse.
No, you cannot - at least not with the default masks.
In principle, the oxygen systems on board can all provide at least 15 mins. of oxygen, in some of the worst conditions (indeed, that's what they're designed to do). However, their sole purpose is to make high altitude air breathable. The key principle here is partial pressure. Basically, the partial pressure of a gas is the fraction of a certain gas in a gas mixture times the ambient pressure. Since normal air consists of about 21% oxygen, the partial pressure of oxygen at sea level is 0.21 atm (atmosphere). If the partial pressure of oxygen is too low, your lungs actually lose oxygen to the atmosphere when you breathe - much like a deflating balloon.
When the cabin pressure drops, the partial pressure of oxygen drops proportionally. In that case, oxygen masks are provided. They do not raise the pressure of the air you breathe in, but raise the partial pressure of oxygen by filling your mask with oxygen. If the cabin pressure would drop to 0.2 atm, and you would be given 100% oxygen, the partial pressure of oxygen would be 0.2atm, which is about the same as at sea level - perfectly survivable. The mask as such does not need to be airtight - it just needs to make sure that there's enough oxygen flowing into your mask to displace all the useless nitrogen in the air. Indeed, the mask you will see on airplanes are generally flimsy, plastic masks.
Under water, these masks are not sufficient at all. They will probably turn into a frothy, bubbly mess of a water/oxygen mixture, especially when the aircraft is filling up with water, due to strong currents. So, you would have to wait for the currents to diminish, and then hold your oxygen mask in such a way that no water will enter it - this probably means you will have to face upwards all the time.
Of course, you can get creative (pull the mask of and breathe from the tube), or luck out with a high quality mask that happens to work underwater. But I'm afraid that as soon as your plane is sinking, it's too late to play the hero, and your best bet would be to get out and at least save your own life.
It is a chemical reaction. So yes, if contained it could provide oxygen under water (provided the reaction did not react with the water). However, it would not work for the reasons already mentioned in the comments and the infeasible logistics of a hollywood captain single-handedly saving his passengers at the last second ;-)
It depends on how much oxygen is provided by the mask, i.e. what the flow rate is.
I guess masks for pilots should be independent from ambient air at any altitude for the case that there's smoke in the cockpit. So yes, they would provide enough air for breathing at sea level, or even slightly under water. But those masks are in the cockpit and not mobile.
Oxygen masks for passengers don't provide all the air they breathe, they just add some oxygen to the ambient air.
Have a look at this diagram, which shows which fraction of oxygen in air at certain altitudes correspond to which at sea level:
It would be fine if passengers are kept somewhere in the yellow region, as e.g. drowsiness wouldn't matter for them, just sitting around.
At an altitude of about 12km/ 40,000ft, the passengers should get air with maybe 50% oxygen, 20% from the ambient, 30% from the mask.
Now, imagine you are under water and try to breathe from the oxygen masks only. You will have to cut off the mask and breathe from the tube. The flow rate from this pipe is just 30% of the usual flow rate, so you have to breathe very slowly. This will be a problem, though the amount of oxygen from the tube would be fine.
Physical stress like diving through the aircraft and pulling out passengers would increase the need for oxygen drastically and then, even the pure oxygen from the tubes would not be enough. You may put several tubes into your mouth, if you manage to do that.
Finally, the gas flow from chemical oxygen generators decreases after activation, and comes to rest after a few minutes, so this wouldn't work if the masks were already deployed at high altitude.
I don't know about the mobile oxygen devices for the crew. If they allow to breathe independent from ambient air, they would work under water. But I guess, they also only enrich the air with oxygen, may be more than the passenger's masks.
In addition to the discussion of the physical ability to use the mask to deliver oxygen under water - it is worth noting that breathing pure oxygen in a high pressure environment results in oxygen toxicity.
In short, oxygen at high concentrations is increasingly poisonous to our nervous system and pulmonary system as atmospheric pressure increases. This makes your example question an even more terrifying prospect if the aircraft is sinking rapidly.
Now, if faced with the choice of drowning or rolling the dice with toxicity, I guess I'd try the mask...