Robot-boat startup HavocAI has built 42 autonomous vessels in 18 months. And now, as the military focuses on deterring China, the company is working to finish a 100-footer by the end of the year.

In an old Soviet-era warehouse just miles from the front in Donetsk Oblast, sparks fly from a welding torch as Ukrainian engineers crouch over the metal frame of a ground robot. Known as an unmanned ...