The deputy director-general of the department, Pornsak Phuimm, said that officials had examined the water quality of 70 rivers around the country from January to March.
Three of the 70 rivers have excellent quality of water, 35 are good quality, 21 are moderate level, while the water in nine rivers is of poor quality, he said.
Pornsak said that rivers with bad quality of water are typically found close to urban and industrial areas.
Before swimming in rivers or lakes, everyone should check if they are smelly or dark, and there should not be any factory sewer pipes nearby, he added.
Pornsak pointed out that there were 16 rivers where fecal coliform bacteria level exceeded the 4,000 most probable number concept per millilitre, which is dangerous for human health.
The nine rivers with poor water quality are: Chao Phraya (Bangkok, Nonthaburi provinces); Nan (Phichit); Yom (Sukhothai); Tha Chin (Suphan Buri); Wang (Lampang); Lam Pao (Kalasin); Songkhram (Kalasin); Mekong (Ratchaburi); Pa Sak (Ayutthaya).
The department also examined the quality of sea water at 77 beaches around the country, he added.
He urged visitors to check the weather before heading to a beach to beware of any poisonous creatures, like jellyfish and blue-ringed octopus.
The top 10 beaches with the cleanest sea water are:
1. Thung Wua Laen Beach (Chumporn province)
2. Haad Rin (Surat Thani)
3. Bang Ben (Ranong)
4. Kai Bae (Trad)
5. Sai Kaew (Rayong)
6. Chaweng Noi (Surat Thani)
7. Hin Ngam (Nakhon Si Thammarat)
8. Pak Bara (Satun)
9. Chan Damri (Ranong)
10. Pak Kor (Phan Nga)