I always dive with a backup computer (wrist-mount, non-AI), so it goes. The main computer is AI, so a mini-spg goes along. I always take a spare mask and a reg set. I have taken 2 wetsuits, one full-length, one shorty, but have cut back lately and left the shorty at home. I always have two lights. I always take a save-a-dive-kit and have used a few things out of it. The only time I took two sets of fins was the time I was testing split fins against my old Quattros to see if I like the splits. The Quattros have stayed home ever since. Fins are pretty bulky and some kind of fins are generally available everywhere. In a pinch, I could make do with something other than my preferred fins. If I am leading a trip from the dive shop I teach at, I will generally bring a spare AIR2, because most of the people with gear from the shop have AIR2s.
But, with current weight limits, I am thinking about cutting back. The first thing to go would probably be the save-a-dive kit. It's really handy, but they often have that kind of stuff around the dive shop, if not the boat, at most resorts. The regs would probably go next. There are always rental regs available. Probably not what I normally use, but something reasonable. I think the backup wrist computer would always go with me. It's pretty compact and light and would save me from missing dives if my primary computer failed for any reason. That means the SPG should also come along. I would also take a spare mask on any trip. Fit is too important and it is too easy to break or lose one.
To answer another part of your question, I don't think any of this stuff is very likely to fail. I had a problem with my AIR2 once, but we fixed it and kept going. By the way, the only gear I bought as a back-up was the wrist computer and SPG. The rest of it is just stuff I have around from upgrading and keeping the old stuff. Once you have it on hand, it's pretty easy to make the decision to drag it along, until weight limits get to be a problem.
But, with current weight limits, I am thinking about cutting back. The first thing to go would probably be the save-a-dive kit. It's really handy, but they often have that kind of stuff around the dive shop, if not the boat, at most resorts. The regs would probably go next. There are always rental regs available. Probably not what I normally use, but something reasonable. I think the backup wrist computer would always go with me. It's pretty compact and light and would save me from missing dives if my primary computer failed for any reason. That means the SPG should also come along. I would also take a spare mask on any trip. Fit is too important and it is too easy to break or lose one.
To answer another part of your question, I don't think any of this stuff is very likely to fail. I had a problem with my AIR2 once, but we fixed it and kept going. By the way, the only gear I bought as a back-up was the wrist computer and SPG. The rest of it is just stuff I have around from upgrading and keeping the old stuff. Once you have it on hand, it's pretty easy to make the decision to drag it along, until weight limits get to be a problem.
