Peter, crossing the sewers using the bridges sounds like a good plan.

If you end up anywhere near a pond on Romney Marsh stop - you're lost. There will be plenty of sewers to follow & cross but also instructions not to attempt to cross other than at a bridge as they vary in width and depth and vegetation and you may well end up sinking rather than swimming.

Sewers are the local name for drainage ditches by the way.

The section of the route across the Romney Marsh will be more interesting if you are aware that the ponds and drainage ditches support the largest British population of the rare medicinal leech: A protected species under Wildlife and Countryside legislation

https://en.wikipedia.org/wiki/Hirudo_medicinalis

Not likely to be a problem to walkers (unless you decide to go swimming with them) but what about the risk to the little leeches with hundreds of walkers tramping through their marshy home?