I haven't yet... had the pleasure of a capsize, however to check your mast for leaks, take it off and put it in the water. Probably easier if you have some help. Assuming the mast is empty of water when you started, you should see bubbles at the places where there are leaks. Turn it around slowly and note where you see bubbles. Of course do this in the early morning when the water is quiet so you can see better.

(The other way is to seal it and when the sealant is dry try the aforementioned leakcheck.)

Then mark the spots with a waterproof marker.

Then take the mast out of the water and let it dry standing upright or upside down, whichever works best.

Then seal it with whatever sealant the next poster tells you to use. I will follow their advice also as I have to seal mine.

To increase your weight to lever the boat back upright, bring a large garbage bag with you and stow it where you can easily get at it when you're in the drink. Simply take it out and allow it to partly fill. Put it over your shoulder and then get up on the boat as per all of the instructions on the Hobie site or the above poster.
You now weigh 10 pounds per gallon of water (in the bag) more, so you have more leverage and your mast is lighter.

(I don't know if garbage bags are class legal.)

The other thing is to put a mast bob on. I'm probably gonna, as the water is cold where I live. I'll just tell everbody it's an autopilot or a radar or somethin high tech that I'm not allowed to talk about.

Hope it helps!