A not very welcome addition to the Romney Marsh flora may have been found at Dungeness today. Floating Pennywort Hydrocotyle ranunculoides is another very invasive aquatic waterweed, introduced to the UK through the water gardening trade. It is reputed to be able to grow 20 cm a day and can form dense mats of vegetation clogging water bodies and causing de-oxygenation.
The plant is similar to the native marsh pennywort but has kidney shaped leaves rather than parasol shaped leaves, and fleshy floating stems with roots emerging at the nodes. The plants I saw today did not form dense mats, but the kidney shaped lobed leaves can be seen clearly in the centre of the picture below, although leaf shape was rather variable.

Hopefully the water body, being shingle based, will be relatively nutrient poor and restrict its growth, but it could become quite a pest, especially if it finds its way into ditches.
NB I have not seen this plant before and the plants I saw had leaves of variable shape, so I am hoping my identification is not correct. For the moment it is probably best to be vigilant incase this species is on the loose. It is easily spread as stem fragments.