When preparing for a dive trip I usually take my own gear but that means a large duffel with BCD, regulators, computer, fins, gloves, booties, mask, spare parts kit, etc.
But there are times I need to travel light due to baggage restrictions or just plain convenience. Most of my items are chosen for reasons of performance, safety, and hygiene. The latter two may be paranoia but why risk it.
My short list of gear fits nicely in a small backpack (well...not the fins) and I rent everything else from the local operator.
3mm wet suit (or equivalent)
Regulator + octo
Computer
Mask
Snorkel
Fins
Booties
Gloves
Small light
Dive log with PADI tables, fish ID cards, etc.
What's in your backpack when you travel light?