A short range model would be useful... perhaps in the 300kg weight range with a flight range of perhaps 80-90km with a more compact seeker... perhaps an IIR seeker with a backup ARH radar sensor. A 30-40kg warhead that was SAPHEI... ie semi armour piercing high explosive incendiary... which means it will punch through the outer hull of the target and explode inside the ship and start fires...
The UKSK model would need a 50kg solid rocket booster to launch it... give it fairly large fold out wings to reduce the required thrust and give it an efficient aerodynamic shape to keep its size down... its targets will not be military generally so it doesn't need to be supersophisticated... in fact a data link sending back IR video of the target would allow the launch platform to target the rudder, or engine area or the bridge if necessary of the target to get it to stop. It could cruise to the target at medium altitude to maximise speed and reduce fuel consumption and when it gets close it could perhaps increase the sweep of the large wings to reduce drag and increase speed and drop down to a sea skimming mode for the last 15km or so to the target.
For military targets such a target would be easy to swat out of the sky, but most smugglers/pirates/illegal fishermen will likely never see what hit them.
You could fit 3 or 4 missiles on top of each other for the length/depth of the UKSK tube system so you could carry 6-8 missiles in two tubes, with the remainder of the tubes fitted with anti sub and anti ship missiles of other types...