Another way of putting it, and perhaps it's only because Ginty is the girliest of the Marlow girls that it isn't said more often, might be that that she's physically courageous: a good climber, confident rider and diver; not so great a batsman, but there is I think a psychological element in Nicola's being able to dolly her out.

Nicola also lacks imagination in her own way: she must be capable of perceiving, with all her naval reading, that stopping the engine runs a risk of running Talisman onto the rocks, but she hasn't the experience to process the real danger of that actually happening.

I think there are other indicators of Peter's dodgy social judgement in canon, though: he doesn't have the sense to see that enraging Edwin is a poor move; his relationship with Patrick is often fraught (though it ends up warm and affectionate, I think). He's clearly capable of considerable charm: managing to fulfil his goals at the Twelfth Night party, fr ex., but I'm not sure I'd relegate his uncertain judgement of character just to a literary device.

Foley hasn't fooled Whittier though, who 'would have had him shot' for the botched raid during the war. I think people like Foley can go on arousing suspicions for years before anyone manages to pin anything on them. (viz. Selby, going as far as to report his unease to the other officer, but not specifying names!)

There's certainly a massive dilettante vibe off Foley, isn't there? Emphasised by his dilettante-wrecker ancestor.