The fire was reported around 7:30 a.m. Sunday.
A witness said he heard what sounded like an explosion, then saw smoke and flames coming from one of the vessels.
Fire officials didn't hear any reports of explosions, but said that it wouldn't be unusual in these types of explosions.
Six vessels were damaged or destroyed - three boats sank, two had fire damage and one boat was hit by a falling mast.
There were no injuries reported.
Fire officials were working with the Coast Guard and the Department of Ecology to minimize the environmental impact.
"While it's undetermined and pending further investigation, we do have some information that suggests that somebody that was out on one of the boats did smell a heavy fuel smell. Shortly after turning on their bilge pump, they noticed some flames showing from their boat. So that may have been the cause of this fire," said Tacoma Fire Public Information Officer Joe Meinecke.
Meinecke said it appears none of the boats damaged had live-aboards.