The group Women on Waves had originally organized the floating clinic to administer the miscarriage-inducing drug for women in Ireland, where abortions are strictly outlawed.
But the spokeswoman said legal and logistical problems had forced them to call off use of the drug. Instead, activists on the boat, which arrived Thursday, will provide information and counseling.
"The complications happened at the last moment and we deeply, deeply regret that this has happened," said Cathleen O'Neill.
O'Neill said about 80 pregnant Irish women had contacted the group about abortions, far more than they could take on.
"We just can't meet that demand," she said. "The second reason is that there are complications with the Dutch law that also mean we can't carry out abortions on the boat." She did not elaborate.
Dutch authorities have said the activists could face criminal prosecution when they return home because the vessel does not have a clinic license.
The boat, called the Aurora, includes a makeshift operating room and has two doctors and a nurse aboard. But Women on Waves said from the start that they wouldn't perform surgical abortions. They planned instead to provide the abortion drug RU-486 to women in the 10th week of pregnancy or earlier, after ferrying patients beyond Irish jurisdiction a minimum 12 miles out to sea.
But Dutch law does not distinguish between surgical abortions and use of RU-486, so the activists still could have faced prosecution.
O'Neill said the group would help the women who had contacted them to get abortions elsewhere.
Ireland, which is predominantly Roman Catholic, strictly outlaws abortion except when a woman's life is in danger. More than 6,000 women left the country last year for abortions abroad, mostly in Britain.
Anti-abortion groups have mostly decided against staging protests near the boat, dismissing its arrival as a publicity stunt.
Just a handful of demonstrators were in sight on Friday, including a man who jumped onto the Aurora with a banner that said "The Irish people in this country have voted against abortion." An anti-abortion boat floated nearby.
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