After doing the post below, a truly evil idea occurred to me. It involves the near-submersibles that drug smugglers use to evade coast guard scrutiny. Those have significant cargo capability.
Imagine one of those with Ukrainian remote steering and live TV installed, loaded with explosives. They are designed to sail awash, but not submerged and have almost no radar signature and little wake.
They are visible in daylight and can be easily sunk by naval gunfire, if lookouts are watching for them. One supposes by the standards of modern weapon systems, they'd be relatively cheap.
An enemy with those could largely "rule" the nighttime sea. I suppose their engine noise can be picked up on sonar but most ships probably don't have an all-night sonar watch.
I hope somebody in our Navy has thought about these; how to use and defend against them.
