It's a personal choice.
In Colorado, many of us have had wilderness training and there is something called The Ten Essentials. They go with you on any hike you do :
- Map
- Compass
- Flashlight/headlamp
- Extra food
- Extra clothing
- Sunglasses
- First aid supplies
- Pocket knife
- Matches
- Fire starter
As individuals we who carry these things have many, many variations on each item and everyone has different specific versions, we learn to find the smallest, lightest versions that meet the criteria so they are not a burden to take along and you won't be tempted to leave them behind. They just go on every hike automatically and you never have to be in a position of wishing you had brought something when you need it.
I only bring this up because in relation to scuba diving, I look at my scuba safety devices the exact same way, in scuba safety there are the 6 essentials:
Whistle
Mirror
Cutting tool
SMB on a reel
Light
PLB
Because of my wilderness training I apply the same logic to these, and have purchased the smallest, lightest versions of each that are acceptable so that I can take them on every dive and will never have to worry about wishing I had one when the emergency happens, because they will simply be there. You have them, you can forget about them and enjoy diving always knowing you've brought along everything you might need to deal with an emergency. Total cost for all of the 6 essentials should be under $400.00.
I hope I am correct in interpreting the "I" as meaning this is what you bring for your diving--not that it should serve as a list of "essentials" for everyone. I mean, someone who dives in a small quarry probably doesn't need a PLB, or SMB and mirror for that matter. It's a fine list for open ocean diving.