Swimming is one of Glenwood’s major sporting codes as well as one of the oldest. Some of the Inter-House trophies date as far back as the early 1930s.
Glenwood has enjoyed 3 golden periods of Water Polo. In the period 1977 – 1982, the 1st team lost only two games out of 95 played during this 6 year period. From 1985-86 40 games were played, of which only 3 were lost. The third period occurred during the year 1997-98 when only 2 of the 37 local games were lost. For these 2 years the 1st Water Polo team were ranked 2nd in South Africa, behind St Stithians.
The school benefits from the involvement of the la Marque brothers - Derek, Steve an Peter - all old boys of the school. Derek la Marque was a Springbok water polo player.
In 2007 the Glenwood swimming coach Ryann Skinner was selected by Swimming South Africa as a national coach to the 12th FINA world championships held in Melbourne, Australia.
In October 2017 the Glenwood under 19 water polo team beat Hilton to win the Top Ten water polo tournament, retaining their title from 2016.