Clyde Morales, of St. Gabriel, La., guided a boat in shallow water as he searched for people in need of help in Humble, Tex.

Calling themselves the Cajun Navy, volunteers with bass boats, airboats and other small recreational vessels set off in a caravan from Baton Rouge, La., on Monday, bound for flood-devastated Houston.

A nine-hour drive delivered the impromptu flotilla to a sheriff’s training facility, where the volunteers bunked for the night. On Tuesday, one group launched its vessels in Humble, Tex., about 20 miles north of downtown Houston by interstate — or as was the case this week, by uncharted river.

With little formal organization, the Cajun Navy has come to the rescue in previous disasters, supporting emergency workers during Hurricane Katrina in 2005 and through catastrophic floods in south-central Louisiana last year.

“We’re trying to do what we can,” said Ben Theriot, an engineer whose house near Baton Rouge was flooded in last year’s storms. “I had people that I barely knew showing up to help me. The best way you can thank somebody for helping you is to go help somebody else.”

Mr. Theriot and a friend, Clyde Morales, piloted a 20-foot aluminum skiff in driving rain. Calls for help were relayed by a…