It should be borne in mind that there is not an awful lot of traffic into the Medway. You have a good chance of sitting there all day and seeing next to nothing.
Thamesport is all but closed, except for a few container feeders; Sheerness is a shadow of its former self. Chatham, Rochester and Ridham are far from thriving. Best choose a day when you know thre is a LNG tanker around. After midday, if it is sunny, the best viewpoint for arriving and departing traffic is from Grain village on the other side