Which is correct?
A.) I wish I was a scuba diver.
B.) I wish I were a scuba diver.
If you chose B, you're correct! Let's try another. Which of these is correct?
C.) If I were president, I would make cupcakes the national dessert.
D.) If I was president, I would make cupcakes the national dessert.
If you chose C, you're correct! Why is that? Well, all the examples above use the past subjunctive verb "mood." The subjunctive mood happens when we are expressing wishful thinking (referring to something that we know isn't going to happen). A few more examples:
If the Easter Bunny were real, he'd probably be kind of scary.
If I were a secret agent, my code name would be The Countess.
If Lynn were here, we'd have enough people to play Balderdash.
I'm not a scuba driver. I'm never going to be president. The Easter Bunny isn't real, and I'm not a secret agent. Lynn isn't here, so we don't have enough people to play Balderdash. All of those are hypothetical scenarios, examples of wishful thinking. Therefore, the subjunctive mood is correct.
So when is it correct to say "if I was"?
"If I was" is an example of the past indicative verb mood. We don't see this one quite as often as we see the past subjunctive, but here are a couple of examples:
If Shawn was at Rob's party last night, he's probably still recovering.
If she was in class yesterday, she heard Dr. Brown's lecture about social cognitive theory.
In these examples, we're referring to things that could easily have happened (we don't know for sure that they did, but they might have). So there's no wishful thinking; the past indicative is correct.
For a more detailed discussion about verb moods, check out this link: https://en.wikipedia.org/wiki/English_subjunctive
