What certifications would you say are a "must have"?

[Imla]

If you live in canada, I would do these in this priority:
1.Drysuit - Water is cold. Being warm = more watertime = more fun
2. Advanced Open Water with: Navigation, Deep, Night, PPB and Nitrox specialties. -> This combo will give you access to the tools you need to be a confident (if you practice) diver in colder/darker/murkier waters and it will grant you access to the benefits and safety factors of nitrox (If you don't ride your NDL) and give you access to most warm water destinations. It will also ensure that you can use that drysuit into the darkness when winter comes. Winter is coming!
3. GUE - Fundamentals: When you have built some familiarity with the drysuit, book a Fundamentals class. It will fine tune your skills and make you an aware, buddy/team oriented diver with skills to keep you out of trouble. (There is no such thing as too early for fundamentals, but do your drysuit and AOW first because it gives you better outcome of fundamentals to have dived a drysuit first, and having deep and night keeps you diving through the year)

 

[_john]

The tldr; Nitrox, deep, drysuit, rescue.

Personally, the "must-haves" were nitrox and deep. Those 2 lift several restrictions that you have as a fresh OW diver. Ironically, they complement each other and exclude each other. Both deal with partial pressures and gas effects, and you can't go deep on nitrox.

The ones that are right up there with them and do not lift any restrictions, but have a significant impact on the flexibility and safety of diving, are drysuit and rescue. Rescue has a lot of very good material and enhances safety significantly. Drysuit makes cold water more enjoyable, and there are a lot of additional issues to consider in drysuit diving that you don't usually experience. For example, floaty feet or a flooded suit. Both are problems, and training helps you deal with both.

I got my AOW through specialties rather than a dedicated course. I bundled several, so it worked out to more of an à la carte certification.

I get the training for the kind of diving that I want to do, and I practice those skills and dive as much as I can.

 

[The Chairman]

AOW & NitrOx.