Duct tape, glue, batteries in every size you use, a bike tube patch kit with glue, not the peel and stick for fixing fish hook holes in BC’s, wings or drysuit.
For those with intergrated 2nds, pack the original inflator in the save-a-dive. IF your 2nd fails a resort or liveaboard can probably rent/loan you a 2nd but then you will also be diving with a strange rented BC and have you seen those rental BC’s!
For international travelers, a copy of your passport main page! That won’t save the dive but it will make getting home easier.