In advance, using royal icing, make 20 blue and 20 rose tip 225 drop flowers with tip 2 white dot centers. Let dry.
Ice sides and background of cake smooth. Pipe-in tip 12 tub area (smooth with heel of hand dipped in cornstarch). Pipe tip 12 top rim on tub, shower neck and shower head. Outline rim of shower head with tip 4 and use toothpick dipped in cornstarch to poke holes in shower head.
Using tip 4, outline bears, pipe in muzzles, inside ears and facial features. Cover bears with tip 16 stars. Print tip 2 message. Pipe tip 4 bubbles on cake top. Pipe tip 4 lines for shower spray. Randomly pipe tip 4 and tip 12 ball "bubble" bottom border. Attach flowers with tip 2 dots of icing.