Thanks Zany for your ideas. Another friend in the meantime suggested that I grow some bulrush in a couple of sides and a few little water plants with some fish, mainly to control the mosquito larvae. I have the bulrush growing in another plastic container at present.
That sounds great also! Please be sure to post a picture of it when it is done. Your wooden tub is really a very nice and interesting piece that will bring you much enjoyment in its second life in the garden.
I must add here that that wooden tub was once used to feed fodder to the cattle that was kept at home much before I was born. It must be at least 60 years old, chopped from a single block of wood. It has cracked near the brim and the slit has gone to the bottom making it unusable to store any liquid as it is. Hence the liner. Yes, will surely post a picture once I begin this project of filling it.