Ok. Flying drones that we've seen in Ukraine lack the range to survive a naval deployment.
Ukraine has had some success engaging Russian surface warships with small aquatic craft type drones.
Does a ground-based operator pose a threat to modern navies by means of some swarm of jet-ski robots?
Yes, but only in the littorals or protected waters. Small unmanned surface vessels can't really operate effectively out in the open ocean. They lack the necessary sensors, endurance, and sea keeping ability.
for now
What does that even mean? There isn't any magic technology on the horizon which will allow small USVs to operate effectively in blue water.
How much does a swarm of extremely long range jet ski robots with hardened military electronics and large explosives packages cost?
Getting enough of anything that can do serious damage to a ship to overwhelm the defenses is going to be an expensive undertaking. Maybe if some idiot sails a $10 billion aircraft carrier close to shore such a mass attack is justified, but it is simply not at all evident that "some success engaging Russian surface warships" equates to posing a serious threat to modern navies.