After my husband built the stand/frame to my specifications - so the bath would be at waist height, no bar across the front (lawn edge), shelf around the edge (explanation below) - we leveled the ground underneath, and laid black plastic down. The base of each leg is sitting on a thick brick tile, to keep it out of any accumulated water. The edges of the plastic are folded up on three sides and secured behind boards used to create an edging that can be weed-eatered up to. On the side edging the concrete deck, it is folded up somewhat less, which means that water can only accumuate in it up to a certain point before seeping out onto the grass.
Underneath, a lack shallow tub (formerly the base of an animal cage on a stand, but it's broken edge means it no longer works for that) was placed. I drilled holes along one end, a couple of inches up from the bottom. This is specifically so it would hold water in it's base, but not completely fill up. It was positioned near the drain hole of the bath above, but couldn't be put quite in position because of the tiles under the feet of the stand, so I attached a cut-down icecream container to act as a ramp to direct dripping water.
I used a bag of stones to line the bottom of the tub, then filled it with compost, and planted some English mint in it - they love damp, shady areas, and cope well with wet feet - I had it growing in pots last year but it never did very well as I wasn't good at keeping them moist. Contained in the tub like this, it can't escape and take over the garden either. And as it grows out the sides a bit, it will smell wonderful when I brush against it.
Later, stones were added to cover the rest of the plastic, and a pot of impatiens was placed in the end, where it's base can soak up excess water that has flowed there.
I then planted the bathtub with two varieties of strawberries, and a chive plant in the middle. When I grew chives in the middle of my strawberry patch a couple of years ago, I have the best strawberries ever! Plus it's handy having this much-used herb close to the kitchen.
I added hoops of 10mm steel rebar to hold up bird netting. My husband put in screws around the sides of the wooden shelf, so the netting is easily hooked over them.
And that shelf around the edge? It serves two purposes - one, it holds the netting out from the edge of the tub, keeping birds off any dangling berries. And second, it's the perfect place to line up small pots of potting mix, and peg down runners that I want to grow roots, so they can later be separated from the mother plants and planted out.