Boaters began calling for help “almost immediately” after the procession for Mr Trump’s re-election got under way on a lake near Austin on Saturday.
Authorities responded to 15 distress calls and received three other reports of boats taking on water, Kristen Dark, from the Travis County Sheriff’s Office, said.
Pictures from the event show the choppy waters of Lake Travis as dozens of boats flying American, Texas and Trump 2020 flags headed out onto the lake.
Ms Dark said weather on the lake was calm, but that the tightly packed boats created large waves.
Officers have found no evidence of foul play, she said, and there were no reports of any injuries.
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.