Getting a group of friends on the water shouldn't require a massive upfront payment. If you aim for a Friday morning, you avoid the weekend surge pricing at the marina. It is the difference between a relaxing day and a stressful bill. You just need to plan the timing of your booking to get the best rate.
Most boat rentals require a deposit or full payment to hold your date. We can use the Citi Simplicity® Credit Card (Citi) to handle that cost. It offers 0% for 18 months on purchases and balance transfers from date of account opening, which makes the math much easier to handle.
This intro window is a firm deadline. If you still have a balance on the card after the 18 months pass, the standard interest rate will apply to whatever is left. That would make your lake day much more expensive than you planned.