Oceanman

Four distances repeat at nearly every stop: 2.5K Sprint, 5K Half, 10K Oceanman and, at one venue, an 18K Ultra. That structure is the appeal. It is a mass-participation open water series where a first-timer and someone chasing the 10K start in the same water on the same morning.

We rate 29 of the 30 at 4 out of 5 for difficulty, and open water is where a difficulty rating earns its place: the distance is only half the question. Water temperature decides wetsuit rules, chop decides sighting, and a calm 5K and a rough one are different events.

Spain hosts the most rounds we index, and the calendar reaches Kazakhstan, Thailand, Ecuador and Peru, which is a wider spread than most swim series manage.

Entry is open rather than qualification-gated. The practical constraint is travel, since these are coastal and lake venues that are rarely near a major airport.