Snorkeling the Great Barrier Reef is best in May or September when the weather is mild and the crowds are thin. You get clear water and comfortable temperatures, which makes being out on the boat all day much more enjoyable.
A trip like this usually runs about $4500. Using the Citi Simplicity® Credit Card, you can pay this off over time. It offers 0% for 18 months on purchases and balance transfers from date of account opening, giving you a set window to manage the cost.
Remember that the 0% intro window is a deadline, not free money. If you have any balance left after 18 months, the bank will start charging interest on whatever is remaining.