I always bring a spare on a trip, I always bring it on the dive when diving solo and often also for buddy dive, but not always. Why. Well, aside of solo where no buddy will guide you up in a failure, I bring a spare mask and other spares to minimize the risk of loosing a dive or several dives on an expensive trip because of a failure of a relatively minor but essential item.
Last (and first) full family of 4 dive trip I brought:
Spare mask for everyone (goes in fin pocket, everyone got 2...)
1 spare 1st stage, one spare second stage, spare inflator, bunch of little stuff (o-rings, tie-wraps, mouth pieces, cord, tape, snaps, split rings, bungee,...)
And silly me:
One extra pair of fins that either me or my son could use and one single extra fin that would work for either my wife or my daughter. Now who brings extra fins and not just fin straps?
Well, I brought my Oceanic Accell in my carry on, but this being the first time pushing a big camera housing with 2 flashes I stuck my more powerful fins in the checked bag when the scale said I could... So I could try ... and my son’s fins my old plastics scuba fins, but only used for snorkeling from the early 90s are visibly on their last leg.
And a single fin?
My wife and daughter both have the same size (XS?) Oceanic Accells. (Great, light, travel fin, if I ever find them for $60, I may get another set for my son). When my wife was doing her OW class in Key Largo,she might have been the clumsiest out of her Element person that ever was not stopped in the pool and still made it out on the boat. She did sort of OK diving wise... on the surface, holding on, trying to get her fins off is when her world came to a crushing halt (then, not anymore). She managed to loose a fin. It was noticed too late, It was gone. No other fin her size could be found. No dive two. Instructor not a ailable next two dives... End of class and trip for her then.
And all that after having made arrangements with a very handpicked instructor running his own show, but, due to sickness getting stuck with well meaning, but overbooked instructor working for the big company... Will never do that again... and will bring spares on a trip...
Now, would I have had a spare fin for my wife on that day boat? No. But you bet that the next trip after I did. Every dive she went... But she’s over loosing fins now...