Unmanned vessel, remotely operated, very similar to the SeaBaby drone that Ukraine has been using to sink Russian warships, although the "blowfish" aren't nearly as sophisticated.
Not sure what kind of explosive they had packed in there, the brief bit of explosion you can see looks very similar to a fuel explosion, so I'm kinda leaning towards ANFO. Ammonium Nitrate and Fuel Oil is pretty easy to make, with the components widely available. I will also point out that they were shooting at the vessel with something quite substantial, as evidenced by the large gouts of water thrown up by the misses. Not sure what, maybe a semi auto .50 like the Barrett, or perhaps the Russian Dshk. Definitely wasn't full auto, or theyd have stiched that boat up good. I have a feeling it was a 50, loaded with Armor Piercing Incinediary. That stuff would be my choice. Anything bigger can't be easily moved about the ship, anything smaller would be near useless against an unmanned vessel.