do you just want a bathtub, or do you want a water feature with plants?
half-burying an old bathtub would be easy, and you would never have to worry about the dogs' claws ruining the water seal. it may be difficult to get any plants but floaters to survive dog visits in such a small area
if you want a real pond (say a minimum of 200 sq ft) so they can run in and out for fun, there are a number of posts here describing different build styles and techniques, and what plants and animals you can choose from. be sure to protect the liner (or whatever water barrier you use) from dog feet