At least 20 people have drowned after a speedboat carrying Indonesian workers home from Malaysia capsized in stormy weather, police have said.

About 90 people were on the boat, which capsized off the Indonesian island of Batam at about 5am, said the island’s police chief, Sambudi Gusdian. He said searchers had so far rescued 39 people.

Police suspect the trip was illegal and that the workers were undocumented because of the high fares that passengers said they paid. Gusdian said an Indonesian woman believed to be responsible for the voyage was arrested as she tried to leave Batam for nearby Singapore.

Haryanto, a 51-year-old survivor, said the boat capsized amid heavy rain and high waves about two hours after it left Johor Bahru in Malaysia. He also said the boat was overcrowded.

“It was so crowded, some of us could not sit,” said Haryanto, who was saved by fishermen after swimming for about two hours. Like many Indonesians, he goes by one name.

A police helicopter and more than a dozen boats were involved in the search-and-rescue effort.

Speedboats and ferries are a common form of transport in Indonesia, a sprawling archipelago. Sinkings are common due to poorly enforced safety regulations. One of the worst ferry sinkings in recent years occurred off Sulawesi island in 2009, killing more than 330 people.