What dive gear do you already have?
Some people find it hard to find a mask that fits their face with minimal leaking. A mask is easy to take in carry on luggage. If you find a mask that works very well for you, and it's not all that expensive, you might want to buy 2. But at least get one. Better to take one you know works on trips, even if you rent other gear.
There's a lot to be said for having your own dive computer, so you learn the interface and understand what it's telling you. Some people like air-integrated computers so it works like an SPG. Some like computers that can log their dives and download that info. to their home computer. Some like wrist units over console units, but if you want air-integration in a wrist unit, you'll be using a wireless transmitter. Whether A.I. function in dive computers is a useless gimmick or a really nice luxury feature is a debate that's been waged on ScubaBoard in various threads.
If you have an unusually large foot size, you may need to buy your own scuba boots and fins. In the U.S., I wear a size 15 shot, so it's an issue for me. If you own your own fins, many people like spring straps; strong, durable, easy on/off.
A piece of dive gear that doesn't always get pushed much at the early stage, but it a good idea for ocean diving, is an SMB (surface marker buoy) and a reel (whether wreck reel or finger spool), so if you're separated from the boat accidentally, you can have a big 4 to 6 foot tall inflated red or orange thing sticking up off the water, making you much easier to find and rescue.
Richard.