"Which of these images appears to contain a stationary obstruction?"

But why the boats then? Are there self rowing boats?

Maybe the next big step is creating AI-driven cargo ships that can independently get stuck in the Suez canal.

On a more serious note, I can't shake the impression that would be a logical next step for all long and medium distance freight, be it road, water, air or space. Whether it's a good or mature idea is anyone's guess.

I wonder if some of the classes are just easy to detect objects that they're using to assess the accuracy of the user.

There sure are: https://www.marksetbot.com/

Boats are on the road too. In the form of being on trailers, Etc.

And don't forget sovereign citizens. They're piloting their vessels using maritime law on every modern highway.

Or sea planes. That was what I was asked for most recently.

Relevent xkcd: https://xkcd.com/1897/

OP is talking about hCaptcha, not google's reCaptcha. Besides, reCaptcha is not being used for that anymore. They probably stopped doing that a while ago.

src: https://www.vox.com/22436832/captchas-getting-harder-ai-arti...