Tides:

www.bbc.co.uk/weather/coast/tides/south.shtml

Weather:

www.metoffice.gov.uk/weather/uk/se/se_forecast_weather.html

www.bbc.co.uk/weather

www.xcweather.co.uk

and if you think the weather at certain regattas might be better for surfing not rowing check out:

www.magicseaweed.com