I was a riverine operator in the US Navy about a dozen years ago, and used to interact with the Customs & Border Protection (CBP) riverine guys from time to time. Back then, they used a green Coast Guard-type slash on their hulls like this:


Source: US Customs & Border Protection website
That seems to have changed sometime after 2014 to a wide blue, narrow red stripe:

Source: US Customs & Border Protection official photo via Wikimedia.org

Source: US Customs & Border Protection Info sheet: "Interceptor Class All-Weather 33-Foot SAFE Boat"
V/r
Jim