A31, Titania, A1 and now Dinky – you must love Fairey restoration!
Failing a local pond you could use to mark the waterline (any of your neighbours got one?)……
As to marking the waterline a dinghy is somewhat simpler than an Atalanta. For the Atalanta I like to level the boat fore-and-aft and athwartships using a water level (clear hose full of water) and then use a laser level set up off the beam and swept from bow to stern. Teamac have a page which neatly summarises the key methods and the great Will Stirling has an article on Classic Boat website.
What do other people do?