Swimming beetle!

[newbiee]

Well, it looked like  a  beetle.

I was emptying out a 500 litre waterbutt today and in the bottom of it (in about 30cm water) was what looked like a huge beetle (4cm across, 2cm wide .. ish) ... I thought it was dead, but it was clinging to the interior of the tap and it swims... it had two little fins/paddles/flippers (type things!) .. it scuttled around the bottom of the butt, feeding presumably.

I could take a picture but I don't think it'd turn out too well.

Any ideas what it is?

[chris pp]

At that sort of size it should be one of the following.

Dytiscus latissimus.

Cybister lateralimarginalis.

Hydrophilus piceus.

Chris