This poem is all about a group of guys who've been living on the water, rowing on a long journey. So they talk about water a lot, and especially about the ocean waves. In fact, the imagery, the sound, and the idea of waves runs all through this poem. Sometimes they seem beautiful and rhythmic, but a lot of times they represent the pain and hard work of life—basically everything that these tired sailors want to get away from.
Line 2: The first time we hear about a wave, it's in the exciting opening lines of the poem. A big wave is building that's finally going to push the tired sailors onto the beach. The description of a "mounting" wave is a nifty way to call up the image of a powerful, turbulent ocean. In general, the ocean seems like a scary and dangerous place in this poem.
Lines 31-2: In these lines, the "wave" isn't a single, literal wave. It's a more general reference to the ocean. The sound of that ocean sounds to the stoned sailors like someone who is either sad or crazy. When a poet gives human characteristics (like sadness) to a thing like the ocean, we call that personification.
Line 45: When the sailors say that their home is "beyond the wave," they are using the word wave to refer to the entire ocean. In other words, they are using a part of something to refer to the whole. The fancy name for that is synecdoche. (Store that up for your next Trivial Pursuit match.)
Line 151: In this case, the sailors use the word "surge" to refer to ocean waves. We love this line for the image it gives us of the raw power of the ocean. Plus there's some pretty great alliteration in there, too ("surge seething").