How do you protect the spare mask from damage? I have crushed a lot of delicate bits in pockets.
Since switching to side mount about 5 years ago, I've worn a helmet, and in addition to protecting my head and holding back up lights, it does a darn fine job of preventing mask loss.
However despite the very low probability of losing a mask breakage is always a possibility, and I'm with James on this one - exiting 2-3K feet on a scooter dive with no mask wouldn't be pretty. Even if you just broke a strap, using one hand to exit with a scooter, and using 1 hand to hold the mask in place, leaves you one hand short to do every thing else. Would it be do-able? Absolutely. But it's a PITA that would be incredibly easy to prevent by just having a back up mask in a pocket.
I don't worry about breaking the spare. I carried a Scubapro Frameless mask in my left thigh pocket for about 5 years, and for the last 5 years or so I've carried a Scubapro Orbit frameless mask. Both of those frameless designs are low volume masks that are also very compressible in the pocket, taking up very little space.
About the only thing you'd break on a frameless mask would be the one piece tempered glass lens. That's not likely to ever happen, but to add a little more security, I carry the lens side of the mask against my leg. Any object that impacts the mask would have to get past the pocket material, the semi-folded mask skirt, and the neoprene slap strap inside the mask with enough force left to break the glass.
In 10 years, of basically abusing and ignoring it in the pocket, I've never broken or even scratched one.
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Someone, back when this wasn't a Zombie thread commented on putting the mask strap under the hood. That works to retain the mask, even if the strap breaks. However in very cold water it gives the water a straight shot to your ears, and that can range from less than pleasant, to cold, to vertigo inducing. I didn't do that more than once or twice in cold water.
Since switching to side mount about 5 years ago, I've worn a helmet, and in addition to protecting my head and holding back up lights, it does a darn fine job of preventing any potential for mask loss.