I really like to see the results of SINKEX. They let you see how effective weapons are. I love them because they support my long held contention, that the way Coast Guard Cutters are armed, can provide us absolutely no confidence, we can reliably, forcibly stop anything larger than small ships.
The nearest thing we have had to a Coast Guard SINKEX did not end well. A firehose, it seemed, was more capable of sinking this poorly maintained, unmanned, derelict small vessel than our 25mm gun.
So far there have been two SINKEX exercises in this year’s RIMPAC. The video above is a series of attacks on the former USS Denver (LPD-9).The second video, below, shows attacks on the former USS Rodney M. Davis (FFG 60), a ship about 10% smaller than the Coast Guard’s new cutters (NSC and OPC), that took place on July 12.
The Drive describes the attacks on the former USS Denver. She was a medium sized ship, displacing about 17,000 tons full load, not huge by any means, but larger than most ships used in SINKEX.
During the exercise, the Japanese Ground Self-Defense Force fired Type 12 anti-ship missiles and the U.S. Army launched guided rockets from a High Mobility Artillery Rocket System (HIMARS) at the naval target from land. From the air, U.S. Navy F/A-18F Super Hornets assigned to Fighter Attack Squadron 41 shot a long-range anti-ship missile while U.S. Army AH-64 Apache helicopters shot air-to-ground Hellfire missiles, rockets, and 30mm guns.
From the sea, U.S. Navy guided-missile destroyer USS Chaffee (DDG 90) shot its Mark 45 five-inch gun. To top it all off, the U.S. Marine Corps joined in with F/A-18C/D Hornets assigned to Marine Fighter Attack Squadron 232 and Marine Air-Ground Task Force 7, who then fired an air-launched cruise missile, air-to-ground anti-radiation missiles, and Joint Direct Attack Munition guided bombs. Other weapons were likely used, but these were the ones disclosed by the Navy.
The smaller, 4100 ton, USS Rodney M. Davis (FFG 60), also did not go down easily. In addition to being hit by laser guided bombs and probably other weapons, she was hit by at least four anti-ship cruise missiles, two Harpoon from Canadian frigate Winnipeg, one from a P-8 maritime patrol aircraft, and a French-made Exocet Block 2 from Royal Malaysian Navy Kasturi-class frigate KD Lekir (F-26).
The two SINKEX were somewhat unique, in that they did not require a torpedo to finish sinking the ships, as has occured in almost every previous SINKEX.
It is true that the Coast Guard is more concerned about stopping ships than sinking them, but getting a mobility kill is very difficult. First the propulsion systems take up a relatively small part of the length of the ship and more importantly, most of it is below the waterline. Unless you can hole the engineroom hull below the waterline, forcibly stopping a ship is almost impossible. Killing steering is similarly difficult. If the target is shooting back, it gets much more difficult.
Chuck Hill's CG Blog 
I remember the Coast Guard had a tough time sinking at least one ship that drifted from the earthquake in Japan – lots and lots of shells
From a stop a ship – wouldn’t it be effective also to take out the bridge? Yes not a large target but tough to steer with shells coming into the bridge – but then probably a secondary helm station – in a way being difficult to sink is a tribute to modern ship building design
In most circumstances you could have multiple video and GPS to after steering and with a little preparation you could make it proof against small arms. In fact there are ways to rig an auto pilot so even if the crew is killed the ship would continue on to its destination.
In 1990 aboard COWSLIP we were ordered to sink a derelict fishing vessel some distance offshore. We put several hundred rounds of .50 cal into her and she would not sink. We ended up ramming the F/V several times before we did enough damage for her to sink.