Short story about an alien bomb

I've been trying to find out the name of a short story, any help will be appreciated:

• Setting (I think) was World War II


• There was a gunboat in the latter part of the story


• The bomb is sentient, and can move around - it also looks like a rock.


• The bomb cannot see water, so it looks like boats are flying.


• It scans boats using radiation, trying to learn about them - which kills a bunch of the crew.


• In turn they fire on the 'rock' - which prompts it to remember its purpose.


• The story ends with the bomb saying something like "ah yes, that's what I was supposed to do" and detonating.

The story turns up in a bunch of anthologies, I just don't have them anymore to check - and google results are swamped by the World War II references.

Many thanks.