The newest boat in the D.C. fire department’s three-vessel fleet sank Wednesday morning while docked at a marina at the Southwest Waterfront Park. Officials said they are investigating a possible mechanical failure.

The loss, at least temporarily, of Fireboat 3, a 29-foot-long boat built in 2014 at a cost of about $300,000, leaves the department’s coverage of the District waterways depleted.

The similarly sized Fireboat 2 was sold, and its replacement is about a month away from arriving, said Dabney Hudson, the president of the firefighter’s labor union; it was confirmed by a department spokesman.

Fireboat 3 is fast, can maneuver in shallow water and is used primarily for rescues. The only other permanent boat in the fire department’s fleet is Fireboat 1, the 71-foot John H. Glenn Jr., which has a steel hull and can break through ice. It was built in 1962 for the New York Fire Department and sold to the District in 1977. It underwent an upgrade in 1983 and is used primarily for fires. It has a slow response time.

Hudson said the department has put a backup Boston Whaler built in 1984, now called Fireboat 4, into operation.

Oscar Mendez, a spokesman for the D.C. Fire and Emergency Medical Services Department, said the waterfront is adequately covered with the boats now in service.

In 2013, under a previous mayor and fire chief, the fire department was hampered by broken vehicles and failed backups. Ambulances were catching fire, and air conditioning often malfunctioned. A new administration has made significant upgrades and bought new ambulances.

[D.C. fire department overstates apparatus inventory]

But Hudson said change has come slowly to the fireboats. Officials have long called for replacing the aging John Glenn boat, but Hudson said estimates for a boat that size are upward of $10 million. A ladder truck, for example, costs about $1.5 million. But the union president said the boat is a “50-year investment.”

“We’re trying to make the effort to fix the problems,” Hudson said. “The longer we wait, the more expensive it’s going to be.”

The fireboats respond to a variety of calls that include fires on boats or on structures on the water, rescuing boaters who fall into the water, drownings and oil sheens. D.C. police have a fleet of more than a dozen small boats for water rescues and searches.