The Supreme Court, in a 5-to-4 decision that highlighted the deep debate among the justices over how far federal courts should intrude into state affairs, ruled yesterday that federal courts will hear cases involving pleasure boating accidents.

Justice Thurgood Marshall, writing for the majority, said federal courts have jurisdiction over suits involving boating accidents that occur on the navigable waters of the United States--even if the dispute involves craft used purely for pleasure rather than for commercial purposes.

In an acid dissent, Justice John Paul Stevens called the majority's ruling an "inexplicable" and "purposeless" intrusion by federal courts into "garden variety" accident cases.

"In my view, there is no substantial federal interest that justifies a rule extending admiralty jurisdiction to the edge of absurdity," he wrote.

The case, Foremost Insurance Co. vs. Richardson, upheld an appeals court ruling in a suit filed by the family of a Louisiana man killed when two pleasure boats collided.

Marshall argued that "the need for uniform rules governing navigation" and the "potential disruptive impact" on commerce caused by pleasure boat collisions required giving federal courts the power to hear such suits.

The dissenters agreed that "standard codes should govern traffic on waterways, just as it is crucial that certain uniform rules of traffic prevail on neighborhood streets as well as interstate highways."

But they said that uniform rules could be imposed without giving federal courts jurisdiction over the nation's more than 14 million pleasure boats. "No one suggests that federal jurisdiction is needed to prevent chaos in automobile traffic, or that only federal courts are qualified to try accident cases," Stevens said.

Who has jurisdiction over disputes involving collisions between pleasure boats is important because state and federal laws may conflict concerning the amount and type of damages a party may obtain.

In the Richardson case, for instance, Louisiana law would have barred the family from recovering damages if the defendant proved that negligence by Clyde Richardson contributed to the accident. Federal law, however, would allow the recovery of damages in that circumstance and reduce damages by the degree of negligence on the part of Richardson.

Chief Justice Warren E. Burger and Justices William H. Rehnquist and Sandra Day O'Connor joined in the dissent.