My wife and I have a modest use of the Spanish language that we used this past week in Cozumel, Mexico. It's always fun being able to interact with locals in their native language.
A big hurdle for beginners is simply learning vocabulary. As such, it is commonly recommended to utilize some sort of flashcard system for learning new words. Many experts, however, recommend a smart flashcard system, like those built around spaced repetition. My personal favorite in this category is Anki, but there are a others out there if you look around. You can read more about spaced repetition at this link.
After learning your first few thousands words, you'll notice that you need to start learning language specific to the things you talk about. So naturally, as a diver, you will probably want to learn some basic vocabulary about diving. Pretty much any dive destination will have English speakers on the boat with you, but think about how suave you'll look using the native language. :)
For me, the biggest hurdle in a flashcard system is making the stupid things! I always have a running list of words to learn, but never get around to making the cards. Ideally, cards should not just have an English translation on the front and target language on the back, but when possible have a picture on the front to really cement the abstract concept.
To overcome my own laziness, and to help other perezosos, I've made a set of flashcards for common diving equipment and generally useful words that can be expressed by pictures. Rather than go for an exhaustive list, I've tried to focus on the common equipment and words that could come up.
I have also included input files for Anki, my spaced repetition program of choice. Just import the text file in the folder and Anki should create the cards for you. These words are Central American Spanish, but should function for most anywhere.
Download the picture / word files at this link.