Boaters began calling for help “almost immediately” after the procession for Mr Trump’s re-election got under way on a lake west of Austin on Saturday, according to Kristen Dark, of the Travis County Sheriff’s Office.
Deputies ultimately responded to 15 distress calls and received three other reports of boats taking on water.
Images of the event show the water of Lake Travis choppy with the wakes of dozens of boats flying American, Texas and Trump 2020 flags.
Ms Dark said that weather on the roughly 19,000-acre (7,690-hectare) lake was calm, but that the tightly packed boats created large waves in areas.
Deputies have found no evidence of foul play, she said.
The first call for help came at 12.15pm, and later distress calls were for boats taking on water, stalled engines and capsizing, Ms Dark said.
Three of the boats that sank were towed to shore, while the other two were still at the bottom of the lake, she said.
Austin-Travis County Emergency Medical Services received no calls for services related to the parade, a spokeswoman said.
