Ransoms of up to $400,000 (£273,000) were paid to gangs which hijacked ships in the Gulf of Guinea in 2015, the BBC reported on Tuesday, citing a maritime document.
The region was said to be the most dangerous in the world for seafarers, with pirates becoming more violent.
A total of 32 seafarers had been kidnapped so far this year compared with 15 in 2015, the report said.
Kidnapping for ransom took place mainly in the oil-producing areas off Nigeria's coast, it added.
The spike in kidnappings appeared to be linked to political developments in Nigeria, the report by the US-based group Oceans Beyond Piracy group said.
There had also been a sharp drop in oil theft last year, which the report put down to improved patrolling of Nigeria's waters, and the fall in oil prices making it less profitable.
According to the report, one of the most high-profile cases was that of the Malta-flagged MT Kalamos, an oil super tanker, which was attacked in February 2015.
The tanker's abducted crew was freed after the $400,000 ransom was paid, the State of Maritime Piracy 2015 report said.