It’s a good thing that "tidal waves" aren't as common in real life as they are in our dreams. Otherwise, we'd all be sopping wet!
Water in dreams is a universal symbol for emotions. Accordingly, when we dream of tidal waves, or of "rising water" in general, we are encouraged to look for areas in our lives where our emotions may be "running high," or even threatening to overwhelm us. Given the background you have provided, it would appear that your dream is no exception.
You have told us that currently you are being pulled in two directions. Your feelings for your male friend are growing, as recently you have begun to think of him romantically. At the same time, you also are aware that your close female friend will become jealous -- and perhaps even angry -- should she learn of this blossoming attraction.
Given the sequence of events in the dream, the first wave most likely represents your increasing feelings for your male friend. You are having an intense discussion with him when you see the tidal wave approaching in the background. It is also significant that, at this point in the dream, you are convinced you both will "die."
Death in dreams almost never should be interpreted literally. Rather, it is a common symbol for change. Accordingly, your dream awareness that you both will "die" most likely represents the transition you already perceive occurring in your relationship. As soon as you recognize this change, however, another tidal wave looms in the distance, "seeming to be from a different source than the first." Do we need a map to locate the source for this wave? It is your female friend's emotional distress should she learn of your blossoming romance.
It's natural for friends to be jealous, and even possessive, with their close friends. Unfortunately, it also is immature and short-sighted. Even though it means her relationship with you will change, your girlfriend needs to recognize the inevitable and give you her blessings, no matter what you decide. It's the only way you two will weather this "storm" together.