You need to get equipment in stages, building your kit up, my logic is what has ruined my dive:
1. A good mask – you can get a decent one for $30-$60 and be sure it will fit you, otherwise you might get stuck with one that doesn't fit right when on vacation which can ruin the dive. You MUST try the mask on in person – you can buy it online later if you like, but you must fit it. A mask is great because its also one of the smallest things you can get and packing is simple.
2. Fins – same problem, I have used rented gear several times and had to use fins with all rubber 'footholes?' that have torn so one or both fins slips when kicking. I much prefer the ones I have bought which come with a strap on the back. Cost is similar to a mask for nice set. These are more of a PITA for packing though, and I don't always take them with me.
3. Dive Computer – I don't have one, just did my first dive (over 80 now) with one in the maldives and thought it was OK – personally, the only thing I need to worry about when I dive is how much gas is in the tank… but the comp does help in making a safe ascent, and warns you when you are staying too deep on a multilevel dive. I don't have one.
4. Octopus/Reg – I haven't bought one, but I have had a lot of crappy ones rented, it does make a nice difference to have your own but for me the price prohibits
5. BCD – I haven't bought one of these either, I have rented several that leaked, but the leaking really doesn't impact the dive, once you buy your own your travel requires another large sized suitcase to accommodate this. Not worth it for me.
I would have the Oct/Reg/BCD if I lived in a place I dived, like Miami, but since I travel and like light then they are too much hassle for me.
The above is the things you are 'supposed to have' The things I have bought, and aren't on the list above are:
Dive Safety Ballon Dive Lights; Dive Knives (I carry 2); Underwater Camera casing; First Aid Kit; Dry Bag (rubber material, buy the big size) so you can fit a nice change of clothes for post dive, a Hoodie or even a T Shirt that are dry are great things on the way back from a dive - anything you walk on the boat wearing will somehow get damp.
GET DAN MEMBERSHIP – they offer insurance for Divers, covering accidents, and even cover things that don't happen when Scuba diving. They will pay for airlifts to a decompression chamber, and the treatment there.