If you are rowing below Marlow Lock, particular care with hygine and boat-washing is recommended because Thames Water have been dumping raw sewage in the river...

 http://www.maidenhead-advertiser.co.uk/News/Areas/Maidenhead/Apology-issued-as-sewage-is-pumped-into-Thames-25042013.htm

The Little Marlow pumping Thames Water Sewage Treatment Works has discharged raw (untreated) sewage into the river a couple of times in recent months.

Whilst there are always micro-organisms present in the river, crews should
be aware of the potential for increased health risks on this stretch and
ensure high standards of hygiene following any outings below the lock, and
extreme care should a capsize occur.

Simon Fisher - Club Water Safety Officer