Hydrodynamics are brutal, but the square-cube law works in your favor. The ocean doesn't care about what's inside your ship. Just the surface of it. The mass is mostly irrelevant. This is why cargo vessels are so large. The bigger the boat the bigger the energy density benefit.
The only real problem is the cost. To compete with a supermax you would need something like two billion dollars worth of batteries to survive a transpacific voyage.
A step before that is to have shore power to run things when in port.
In general stopping pollution near populated areas and busy ports is a bigger win and so a good place to start.
However, feeding the coolant through some sort of heat exchange with the sea water is very likely something they do.
Aren't ship engines usually cooled with sea water already (through a heat exchanger, not directly)? So it's not really something new
The only useful picture in the article is a "solar powered board", which doesn't give a good scale but appears to be barely enough solar power for the rich owner to enjoy their luxuries and will barely move all day while forced to anchor all day. I'm sure it can go really fast on battery power, but not for long. Sails would be a better primary movement source with battery for in harbor navigation or when the wind stops backup only.
I'm imagining channels in a busy waterway or a regular ferry route. Of course it needs economy of scale, but at that point everyone saves on batteries, etc. Unless there are submarines, I don't know what would pass under the boat and cut the connection.
Maybe watercraft usage or routes aren't predictable enough? Maybe the sliding connector is too expensive underwater, especially in sea water.