Iceberg (iceberger) — a person who makes a practice of swimming regularly during the winter in unheated water.

I’m a regular swimmer myself, right in the middle of winter. They reckon I’m the oldest iceberg in Sydney (G. Lehmann, Spring Day in Autumn). Just when I thought it was safe to dive into Brighton beach as an Iceberger, a grey ghost was circling (Age [Melbourne], 2011, 15 Apr., 16/7).