Blue Lagoon

It is busy and it is worth it, once. The water is milky blue from silica, the setting in the Reykjanes lava is unlike anywhere else, and it sits twenty minutes from Keflavík Airport, so it fits an arrival or departure day. Book a time slot in advance; it sells out. If crowds bother you, go early morning or after 20:00. The Secret Lagoon or Fontana on the Golden Circle are calmer and cheaper alternatives.

Yes, and it is the best way to use it. Transfers like IceTransfer's Keflavík to Blue Lagoon to Reykjavík (/experience/1036423/) or the reverse (/experience/1036419/) stop at the lagoon for a few hours with your luggage stored. Allow at least three hours at the lagoon and check that your slot lines up with your flight. Note that the Reykjanes eruptions have closed the lagoon at short notice; check its status before travelling.

The Secret Lagoon at Flúðir is a simple, warm geothermal pool with a bubbling hot spring beside it and basic changing rooms. It is on the Golden Circle, so it slots into a sightseeing day (/experience/17050/, /experience/624446/). The Blue Lagoon is a spa: silica masks, swim-up bar, saunas and a price to match. If you want a soak after a day of waterfalls, Secret Lagoon. If you want the spa day, Blue Lagoon.

Yes, at every public pool and most lagoons. Swimwear off, wash thoroughly with soap, swimwear on, then into the water. It is a hygiene rule because Icelandic pools use very little chlorine, and it is taken seriously. Changing rooms are single-sex with open showers; some larger lagoons have a few private cubicles. Nobody looks. It feels odd for thirty seconds and then it does not.