Exactly, something the size of a speedboat. It's why Ukraine launched a raid at the Sevastopol port at night when the Russian guard is down. An ongoing vessel will be constantly on guard and will need to spam the defenses to get even one hit in. And that one hit isn't guaranteed to be lethal to the ship (as seen with the Tanker War & the USS Cole) unless it's fire retention, crew training, & maintenance has been god awful as it was on the Moskva.