Help the daydreaming student complete their thought during a terrible lecture.

This math lecture is endless, and it's freezing in here. If only I ___ on a tropical beach with a coconut in my hand!

The correct answer is were.

After "if only" for present unreal situations, we use the past simple. For the verb "to be", "were" is the grammatically correct form for all subjects in hypothetical situations (the subjunctive mood).