In the 1940's, before the nuclear age, during World War II, some recently declassified documents depict something extraordinary: military scientists experimented with sending tsunamis into the Japanese coastline using explosives, to wipe out those civilians who were defending the coastline against an invasion with pitchforks and keep the human cost of an invasion down. The plan was shelved after the military scientists determined that it would take some 2 metric kilotons of explosives to create such a wave... at least if those explosives are placed in a straight line.
One thing that really intrigues me about how tsunamis can be focused, from a fluid dynamics standpoint, into a relatively small area, is the Cascadia earthquake and tsunami of 1700. According to computer models, there's one portion of the wave, as it leaves Cascadia, that's significantly larger and more destructive as it travels across the Pacific than any other part:
Note how that portion of the departing tsunami, which appears to also have a dent in it, almost conforms to the shape of the subduction zone that created it: all that tsunami energy appears to be focused on that one point, where the wave is both longer AND taller than it was in Cascadia. Could explosives do the same when placed in that concave pattern?
Despite how far-fetched it may sound, accidents involving man-made explosives have created tsunamis before. Take the incident in Halifax in 1917, for example. The SS Mont-Blanc, a cargo ship about 1.5 times the size of your average jumbo jet (which is not very big for a cargo ship, let's be clear), sailed across the Atlantic, loaded from bow to stern with military high-explosive cargo, in the form of mostly nitrocellulose, TNT, and picric acid. When the Mont-Blanc got to Halifax Harbor, however, she was broadsided by a Norwegian ship, the SS Imo, and caught fire. That fire then went on to ignite all those explosives at once. The resulting blast had the force of 2.9kt of TNT, which is just about as much explosives as Project Seal would have needed to be effective ― and it generated a 60-foot tsunami that devastated the portions of Halifax not already blasted away by the explosion itself.
Fast-forward to today, and we have technology that absolutely no one dreamed of back then. We have computers that can fit in our pockets. We have cars that drive themselves. We have unmanned, remote-controlled aircraft that use cameras to tell their remote human controllers where they are going ― even ones that can attack. So why not also use that same drone technology to remotely navigate cargo ships the size of the Mont-Blanc filled from bow to stern with explosives ― about 10 of them ― into a V-shaped pattern with overlapping blast-radii, then place remote-controlled detonators on them, along with "Fire" buttons on the remotes?
The overlapping blasts would displace a lot of water, to be sure... but then the water has to rush back into that V-shaped depression (in contrast to the linear depression that the military was thinking of creating off Japan during WWII... and also in contrast to the circular depression that was created by the Halifax blast) that the blasts leave behind. The result? Massive drawback... which is most powerful on the concave side of the shape. The wave follows, refracting into a 200-plus foot monster at the very least, the way the water flows towards it... in fact, if this is done in a very deep section of ocean (even if mostly landlocked), it may reach as high as 1000 feet or more, thus becoming a mega-tsunami as it is focused into that V and directed towards its target, at which point, because of the way it is refracted, it should easily be able to cross an entire ocean (or sea) towards the enemy in just hours, or even minutes if the ships are blown up close enough to the enemy in question.