What may an open water swimmer wear and carry?
Officials' detail · The competition · 2 of 2A swimmer may use an approved swimsuit, goggles, up to two caps, a nose clip and earplugs. Nothing that aids speed, endurance or buoyancy is allowed. Watches and jewellery must come off, and nails must be trimmed. The race number is written on the arms, upper back and hands. Sensors may send data out but may not feed it to the swimmer.
How it works
- Allowed: "Approved swimsuit, goggles, a maximum of two (2) caps, nose clip and earplugs may be used."
- Banned: any device which may be an aid to speed, endurance or buoyancy.
- Nails and jewellery: the clerk of the course checks that all competitors "have trimmed fingernails and toenails and are not wearing any jewellery, including watches".
- Numbers: on the arms, upper backs and hands. On the arms and back they are 100 mm high and 60 mm wide.
- Team caps: in the relay, all members of a team wear the same caps.
- Technology: GPS tracking and sensors for things such as stroke rate or body temperature are permitted when approved by World Aquatics.
- One way only: such technology "may transmit such information but not act as a receiver giving information and advantage to the competitor such as 'smart goggles', hearing devices, and the like".
- Declaring: wearables must be declared in the first call room. Failure to declare may result in disqualification.
- The escort boat: shows the competitor's number on both sides and the flag of their federation.
Tricky cases
Why do swimmers have numbers written on their arms?
So that officials can identify them. The rules require numbers on the arms, upper back and hands.
Can a swimmer wear a sports watch to check their pace?
No. Watches must be removed, and a device may not give information to the swimmer.
Can a swimmer wear two caps?
Yes. Up to two caps are allowed.
Can a swimmer wear goggles that display their stroke rate?
No. Smart goggles are the rule's own example of a device that is not permitted.
Why the rule exists
Trimmed nails and no jewellery are for the other swimmers: in a pack, a sharp nail or a watch strap cuts. Numbers on the skin are how referees identify a swimmer from a boat when every cap looks alike. The ban on receiving data keeps the race in the swimmer's own head.
Where it differs
| Where | What changes | Source |
|---|---|---|
| World Aquatics rules | As above | OWS 5.10; OWS 7.4; OWS 7.5; OWS 8.1; OWS 3.15 |
| Pool swimming | Suits end at the knee. A data-collecting device is permitted if it sends nothing to the swimmer | Swimming Rules 14 |
| Cold water | Wetsuits become compulsory below 18 degrees | OWS 7.2 |
From the rule book
World Aquatics, Competition Regulations, Part Three: Open Water Swimming Rules (in force as from 1 January 2025), Articles 3.15, 5.8.1, 5.10, 7.1, 7.4, 7.5 and 8.1. Checked 14 October 2026. Official document: World Aquatics Competition Regulations (PDF).