What can break? Lets see:
1) UJ (common)
2) Mast (not uncommon)
3) Booms (not uncommon)
4) Fin (not uncommon)
5) Down haul line (not common and not critically disabling)
6) Sail top fixing can fail (uncommon)
7) Sail tack fixing can fail (uncommon)
8) Sail panel can fail (common but not critical)
9) If you sail a Bic your mast track can fall out. (uncommon but critical)
10) Board can snap but this is uncommon
These have all happened to me except the board snap.
What to do?
1) Regularly check your UJ for flaws. Ensure your safety line is up to the job. Some are too thin and snap. Rubber Boge UJs can fail without any warning due to internal corrosion in such a manner that the safety straps don't work. To be safe don't use them or, if you do, then replace them every year. Tendon UJs crack at the bolt holes. You have to bend them hard to inspect them.
2) Make sure when you rig that your mast ferrule is fully inserted. Taping it up is good practice. That keeps the sand out too and stops it getting stuck together. Apart from that you should be aware that light weight race masts are particularly fragile. 70% to 80% carbon masts are more durable than 100% carbon and skinnies are more durable than standard diameter masts. In the event of a breakage you MAY be able to remove the top part from the mast sleeve and put it back in upside down then fold the top of the sail over and tie it or just let it catch on the jaggered end of the mast.
3) Aluminium booms die after a few years and fail usually at the head without a lot of stress. Carbon booms have an indefinite life but when they fail they fail catastrophically. Usually only one side fails so you can get back even if you have to take the booms off and turn them over.
4) If you lose your fin you can still sail if you practice sailing fin first. This is a useful skill for other reasons. It is reasonably strenuous and getting upwind is a chore. If there are waves it's almost impossible.
5) You can replace a broken downhaul line on the water but achieving proper downhaul tension is difficult. The North XT extension makes this easy. Or you can carry a tug cleat. I usually do in case I want to adjust my dh while I'm out. You can still sail with inadequate dh though so you will get back.
6) If the sail top fixing fails the sail falls down the mast and handles like a tarp in a wind. You really have to be able to tie it back up. How you do this depends on the way it fails. This can be difficult and finicky. As long as you still have a pin or cap for the mast tip fixing you should be able to jury rig a tie with the help of a knife with a fid.
7) Tack fixing ditto but not as critical as a top failure.
8) Sail panel failure is not critical. Just sail home and tape it up.
9) If your mast track falls out (unlikely) or you have a board and rig separation for any reason (not uncommon) stay with the board. If you can mark the position of your rig or attach your pfd to it without letting go of the board then do it and paddle back. Don't let your board float away. Without a rig attached you will not be able to catch it in any but the lightest breeze.
Some things cant be fixed and you will have to self rescue. The recommended procedure is to lay the rig over the front of the board and lie on top of it with your legs through the booms and paddle back fin first.
If you snap your board I guess you keep the biggest bit to get home on
. Thankfully I've never had to use it on the water, but have had two downhauls break whilst rigging and it's come in handy then.
