A boat trip to Rottnest Island was short lived when a vessel containing six people overturned in the Swan River.

The small boat was under the Stirling Highway bridge at Fremantle when it capsized before reaching open water.

The small boat was swamped by waves from other vessels.

Jayde Papas was onboard the boat and said it was a scary experience.

"We were just going to Rotto, and [there were] a few too many boats coming in to go under the bridge," she said.

"Two-metre waves came over the front of the boat and pretty much took us sideways and flipped the boat.

"Very scary, the most scariest thing I've done in my life."

Ms Papas said everything in the boat had been lost.

"The boat's upside down, bit hard to flip it back over when the boat's full of water."

No one was injured in the incident, and other vessels quickly came to their assistance.

A sea rescue boat and water police were also on the scene in minutes.

The boat was travelling under a bridge when it capsized. ( ABC News: David Weber )

Water safety conference just up the river

Sunday's incident happened only a short distance from a Department of Transport media conference on boating safety on Australia Day, typically the busiest day of the year on the water.

Department officer Luke de Gracie called on all boat users to be aware of closures and restricted areas, and to make sure they were properly prepared.

"They should check their safety equipment, they should check their navigation lights and ensure that they've got spare globes and fuses for these," he said.

"We also recommend that you bring a suitable anchor and lifejackets for everyone on board."

Mr de Gracie said there had been problems with people not displaying the correct navigation lights in the past.

He also urged skippers not to drink alcohol.

About 10,000 boats across the state are expected to hit the water on Australia Day, with up to 1,000 vessels in the designated Swan River spectator areas for Skyworks.