Following on from ANHBUC, it's a good idea to have some shallow shelving in the pond at the sides to help frogs etc get out as they can drown. Bit difficult with a bath but you could put a plant in a container (weighted down) in the middle so it is like an island just at or below the water line and then have a ramp from that to the edge of the bath. Something like a small water lily or an iris are quite good but these can tend to take over so you will need to keep them under control. Also try to put some plants around the edge as cover. The foliage is good for the fauna to hide in and under - it might be a good idea to get some floating weed as well (any garden centre with an aquatic centre sells this). The frogs will like basking on the island when it is warm weather and they seem to quite like shallow water to lay spawn in.
Tap water is ok as long as you let it stand say overnight before you put any fish etc in. They say that fish and newts and frogs don't go together in a pond as the newts and fish tend to eat everything but my pond is choc full of them! You could see if anyone you know has any pond flora/fauna they want to donate as lilies, water soldiers etc always need thinning out and water snails are always quite useful - I did a fish for newts swap with a friend who breeds goldfish (unprotected newts I hasten to add). Also a couple of pints of the black gunk from the bottom of an established pond is great as a quick starter for a new pond - smelly but let it settle! But you will find all sorts of things takes up residence after a while and the wilder it looks the better it is for wildlife so don't be too clean in terms of fishing out dead leaves.
Once it has started to establish, go out when it is dark with a torch and you will see all sorts of things in there that you won't see during the day. It gets quite addictive!