] would Darcy not have emerged from the pond covered in ticks and leeches? Below is an interesting note re Lyme...

I have bathed in rivers, lochs and in the Hampstead "bathing ponds" without ever getting a leech. Ticks are picked up in long grass etc where they await hosts, leeches lurk in the water but neither are often a problem in England.

They may have been more common in JA's day, of course. Medicinal leeches are now a protected species - but pollution seems to be the main reason for their deline. As an important element of Regency medicine I imagine that they were valuable and enthusiastically collected so I doubt that there were masses more in most ponds in Darcy's day!