noun - 5-wide (Five-wide) is a formation of five open water swimmers or triathletes swimming side-by-side during a competitive race, a tandem swim, or a training session.
There was a 5-wide coming around the last turn buoy.
Its equivalents (3-wide, 4-wide, 6-wide, 7-wide, 8-wide, etc.) are used when there are more individuals swimming side-by-side.
Usage dating to circa 2001 from Steven Munatones in Huntington Beach, California as adopted from the modern-day application of a NASCAR racing term.
with Darren Miller, Jamie Patrick, Jen Schumacher and Oliver Wilkinson across the Strait of Gibraltar
Six-wide at 2010 USA Swimming National Open Water Swimming Championships
In Long Beach, California