 It often occurs in stagnant, warm bodies of water and can also take on a rotten smell or make water look like pea soup, says Fettgather. If a human comes in contact with or ingests harmful algae, common symptoms include nausea, vomiting, and skin irritation or rash. Fettgather also warns of the increased danger for dogs, who are more susceptible to the toxins in the algae. They are also more likely to ingest water when swimming.