aren't squid usually damp?

The usual phrase, of course, is damp squib -- but the mental image of investing £600 in a damp squid is sufficiently arresting that I think we should promote this wording as an improvement. It's like the way the phrase used to be "as large as life and quite as natural" before Lewis Carroll improved it to "as large as life and twice as natural", and it's only ever the improved version you hear nowadays.

Peter