Notice that the first phrase is in the subjunctive mood, "if you were to wish." The poet might have written, "if you HAD wished", which would have made us understand that he was talking about something in the past, which might have been done differently under other circumstances.
Or, the phrase might be, "if you wish", indicating a present moment. In other words, "I'll do this right now if you want me to do so." However, the subjunctive indicates that the suggested activity did not occur in the past, and may or may not be occurring at present. It is also not certain that the wish WILL be the case in the future. The poet gives the loved one extreme latitude. He might have written "if you decide you want me to...", or some other phrase to indicate that the decision about what is either to happen or not to happen is totally up to the person being spoken to.
What does the word, "depict", mean? It comes from the same root as our word, "picture". Therefore, the poet is promising to make their love known in a very concrete way, just as would happen if we were to be able to take a picture of love, show it to someone else, and say, "see, here is our love".
Its up to us to use our own imaginations as to how this depiction of love might take place. It could be through writing a poem or a story, drawing or painting a picture, or simply writing "I love Susanna" (or whatever her name might be), perhaps in the way graffiti is done today. The poet does not give specifics as to how this is to take place; that is up to us, as the reader, to decide.
When the poet promises to depict their love for all the world to see, it is obvious that there is to be no secrecy about it. He doesn't want to withhold the fact of their love from anyone at all. Instead, he wants to declare it for all of mankind to understand. To further emphasize the grandeur of his love, he mentions both the height and depth to which he is prepared to go. For the poet, the deepest and lowest geographical place is the bottom of the sea. Therefore, he promises to "depict" their love to that great depth.
Similarly, the poet cannot imagine a geographical location which is higher than the peaks of the mountains. Thus, he also promises to declare their love to that location. We can also understand that the promise is not only to depict their love at those very locations (the depths of the sea and the peak of the mountain), but to everything in between. Therefore, there is no limitation to the places where love will be proclaimed.
The imagery leads us to understand, in addition, that the mountain peak and seabed are simply indicative of the encompassing nature of the love. That is to say, there is NO limitation to his love or to where he would make it known.