And no, I'm not clever for figuring it out, it's just that GRRM does seem good at setting out the logic of the strategy. This is not a book where the plot is driven solely by characters making stupid mistakes and self-destructive decisions (though there is some of that). A lot of the plot does seem to be driven by the logic of strategies at play.
Anyway, Davos figures it out about two paragraphs after I do, and sure enough the chain comes up, the ships burn from wildfire hurled at them with trebuchets, and Davos is plunged into the water and swept downstream towards the chain and a lot of burning ships. Cue ad break.