Apart from the typo it seems much more personal than the normal London Underground "Stay Cool" or "Carry Water" posters.
I particularly like the request to "Have a magical weekend my lovelies" and the little PS to ask staff for water if needed.
Other signs that summer's arrived on the Tube - along with the outbreak of sandals & sunglasses - are the stream of announcements about delays due to people who are ill on trains. Not sure if water is a general cure all to stop commuters fainting in the heat & perhaps if water was more readily available, that might help trains being delayed as a result of people swooning.
If you see (or hear) of any other signs of summer on the Tube or your station has any recommendations of how to deal with the heat, let me know!