Drill a few small holes in the bottom of the drawers of a set of plastic drawers. Use to store bath toys, just put them away and leave in the tub until the water drains away then store away in the drawer unit.
Take a small paddling pool on holiday- use as a baby bath, fits in the shower tray in caravans etc. I bought a couple for 50p each at the end of last summer
Use a flexi plastic tub as a baby bath, they get a nice deep bath without wasting water, and mine fits in kitchen sink so saves my back.
Store the instructions for your pushchair, raincovers etc in the Boot of your car.
Use towelling dressing gowns/bathrobes as swimming towels. Wrap up little ones, get dry yourself, hey presto kids are dry too!