No to water retention tablets I'm afraid.
Unless it's a big problem for you, in which case you need to see the docs, try not to be unduly concerned about it.
Sometimes when fat leaves the fat cells, they can fill up with water. It's only temporary but can show a plateau on the scales. This has nothing to do with fat loss and one of the reasons why you need to focus on what you do between weigh ins rather than what the scales say.
Anyway, at some point, the water does a whoosh and the scales will drop. It's all very normal.
In the meantime, make sure you are drinking adequate water throughout the day, and as I say, if you are really concerned, take a visit to the doc, but do tell him/her you are on a ketogenic VLCD diet as they are naturally dehydrating anyway (hence the water requirements)