I've just been for a swim in the Thames....

[El Gordo]

By the way, I notice that online just now are me and 27 'guests'. It's really quite a good party.
As it's before 8 am on a Sunday I guess they're 'spiders' or some-such internet-type auto-checking device (losing touch with technology here).
What are these critters, and what purpose do they serve?

That's probably because I checked a box under the hood last night, thinking "I wonder what that does? I think you just told me. It makes their activities more visible. As for what they do, they are mostly search engines looking to bookmark pages. I think. I don't really know.