Ontario

The sea in Ontario peaks in August, when the water averages around 22°C.

The main swimming season runs from July – August, when surface temperatures stay comfortable for extended sessions.

On average, 2 months a year sit above 20°C — the threshold most swimmers describe as pleasant without a wetsuit.

Shoulder months like June and September are noticeably cooler but often quieter, with fewer crowds and steadier weather.

Outside peak season the water drops toward 7°C, so a 3/2mm shorty or full wetsuit makes long swims much more comfortable.

The coldest month is typically February at around 0°C, when cold-water swimmers dominate the coast.

Long-term climatology for Ontario shows an annual mean sea temperature of 10°C, ranging from 0°C in winter to 22°C at the summer peak.

The seasonal swing is roughly 22°C between the coldest and warmest month — a useful yardstick when comparing years or checking whether the current sea is running above or below average.

Recent decades have trended slightly warmer at Ontario, in line with regional sea-surface temperature records for the wider basin.

Monthly averages below are drawn from multi-decadal climatology and are updated as new observations arrive.