A lady I know was boating on the Norfolk Broads very recently and given this advice, which I felt worth passing on to all dog owners:
Lakes, ponds and rivers have Blue-Green Algae at the moment.
This is highly toxic for dogs, so we would advise you to avoid walking in these areas at the moment, or keep your dogs on leads to prevent them going in the water.
Every year dogs die as a result of swallowing the algae when in the waters that have it.