They're showing the mystery antenna there.
With only three whip antennas seen on the vessel, and assuming that like most commercial & public safety vessels, they have at-least two VHF Marine radios in the bridge (one to stay on 16, the other for use on a 'working' channel), that third antenna
should be for their agency/mission radio, in this case, the CBP VHF net. Then ideally, their handheld radios are APX7000 or 8000s that can do VHF Marine, VHF CBP & other agencies, plus 800MHz MPSCS.
Years ago, when I lived on the 23rd floor of Riverfront Towers apartments (100 W Jefferson Ave) along the Detroit River in downtown Detroit, the Border Patrol boat (sometimes with USCG) would routinely hide in the marina in front of my apartment complex at various hours, either watching a suspicious vessel or loitering in-wait of anything suspicious. I first found that out by getting an Optoelectronics freq counter hit on their old 163.625MHz VHF repeater input, but later would see them up close as I was hanging-out on a friend's boat in the marina -- we'd gently tease them -- offering them beer, etc. & they were pretty friendly.
I expect he's finally retired by now, but back in the 1990s/early 2000s, an agent in Detroit Sector with callsign "BP-206" was a big-time radio geek and pushed to make sure he in-particular & others had the right freqs programmed into their radios, and he wasn't shy about jumping-up on someone else's radio net. Crazy guy would even back-up DPD 1st, 3rd & 4th Precinct units on hot calls.
The two attached photos were taken out of my living room window.
