@Audrey 2 - a tin bath makes a very good pond indeed (I know, I've got one). You could sink it if you want to, but you don't need to, my frogs are happy climbing into it via the wall trellis or the pile of stones I've laid against the side of the bath. But if you do sink it, it will be less likely to warm up - and therefore dry out - in summer. Do let us know how you get on.
@SarahH2 - yes it could be worth wrapping the potted ponds in winter, as the containers could break in freezing weather (especially if terracotta). They'd be too heavy to move into the greenhouse though!
@Esvery - I have a shady garden, too. I've got brooklime, frogbit and hornwort growing in my pond. You could also try water forget-me-not, which is very robust, though it might not flower as much in really shady conditions.
@Marygold - any body of natural water in a garden will be used by one creature or another - I regularly find my frogs in the bird bath! You can make a decent pond using an old tin bath or Belfast sink - which may only be one-metre-squared. If you grow submerged, floating and emergent plants in it, you should attract a range of water creatures, such as frogs and water boatmen, even the odd dragonfly if it's sunny.