LONDON (AP) _ An inquest opened today into the sinking of a pleasure boat on the River Thames, and new safety regulations went into effect. Police raised the death toll to 26 after another body was discovered.

Police said 31 others are missing and believed dead from the accident Sunday, in which the Marchioness was rammed by a sand-and-gravel dredger, the Bowbelle, and 79 survived.

Earlier, police said 37 were missing and 87 survived.

The Transport and General Workers Union said crewmen based at Westminster Pier, near the Houses of Parliament, decided at a mass meeting early today to cancel all pleasure boat sailings until the wreckage is removed.

''The men feel very sad about what has happened and they feel they just can't operate while the Marchioness wreck is still lying there,'' said union spokesman Michael Cashman.

The latest body was found near Battersea, about 2 miles upstream from the wreckage, police said.

The Marchioness was resting on a sandy bank at the riverside near the accident site, exposed to view at low tides.

A young crowd of guests from the worlds of fashion and banking were partying aboard the 90-ton pleasure boat, which sank in the fast-flowing tidal river within a few minutes.

At a London coroner's court, the inquest into the 16 bodies which have been identified opened briefly and was then adjourned by coroner Dr. Paul Knapman. It will resume after other inquiries are completed. Under British law, the inquest had to open before the victims could be buried.

Two new regulations for pleasure boats on the busy Thames went into effect today. They were announced Monday by Transport Secretary Cecil Parkinson.

One covers checks on passenger numbers. Captains are already required to count passengers to ensure the boats are not overloaded. Now they will have to leave a record of passenger totals at their headquarters before sailing.

Tidal Cruises, the company that owns the Marchioness, has denied the vessel was overcrowded.

The vessel, hired for the 26th birthday party of London financier Antonio Vasconcellos, went down shortly after leaving on a 1 a.m.-to-dawn trip.

The dredger struck the Marchioness from behind and then ran it over, shearing off the pleasure boat's top deck. The wreckage carried underwater scores of people who had been drinking and dancing below deck.

Vasconcellos and the captain, Stephen Faldo, were among those still missing.

The other new regulation says pleasure boat passengers must be given more information about what to do in an emergency. Up to now, passengers have not normally been given any instructions about escape routes or safety equipment.

A Scotland Yard spokeswoman said Monday that police investigators are considering bringing criminal charges over the disaster. She said charges of ''unlawful killing,'' roughly similar to manslaughter, and other related charges are being considered but she refused to elaborate.

Police on Monday said they had released the dredger's captain and second mate, who were arrested and questioned after the sinking. They said they also interviewed the five other members of the dredger's crew.

Along with police, the Department of Transport's Marine Accident Investigation Branch and the Port of London Authority were investigating the accident.

The Independent and Evening Standard newspapers on Monday reported a disagreement had developed between the two captains over which vessel had the right of way under the central span of Southwark Bridge, where the collision occurred.

''It seems that both captains thought that they were in the right, but technically the actions of both could be held up to scrutiny,'' the Standard quoted an unnamed river authority source as saying.

But Jane Carmichael of Tidal Cruises said that although smaller craft usually stick to the side arches, Faldo was correct to move through the center Sunday because of tide conditions.

''We do not know what went wrong,'' she said. ''Our captain was acting totally correctly.''

Police said they listened tape-recorded radio traffic between the captains of both vessels and shore control for clues into the cause of the crash.

Scotland Yard declined to comment on the newspaper reports.