Wear comfortable hiking clothes, and I'd recommend wearing a bathing suit under your clothes if you're planning to swim. The changing rooms are primitive and there are no showers or bathrooms. Bring towels if you plan to swim.
The weather in Iceland can change quickly! It’s best to prepare for this by wearing layers (with a waterproof outer layer).
The pool at the end of the hike has been featured prominently on social media, so early mornings are best to avoid potential crowds.
This is a great activity for when the weather is nice. (In Iceland, it can be sunny one moment and cold and rainy the next.) Once you start hiking, there isn't any shelter until you reach the pool. You don't want to be caught in bad weather!