Dim the lights and turn on some very soft music. Barry White is always a good choice. Find as many candles as you can and put them all over the place. Tealight candles work great and are inexpensive. The more candles you have, the better the effect will be. The light will bounce off the water and the shadows and movements are beautiful. Be careful not to place the candles where you will start a fire and remember to leave room to get into the tub!
All in the Nose
Go for the sense of smell. Start with flower petals in the water and around the bathtub. Find some bath salts and essential oils like rose or lavender to put in the water. After that, get some potpourri and put it around the room. Tiny pretty perfume bottles, placed stategically all over the bathroom along with candles, will glow and glisten in the candlelight. Be careful not to mix scents that don't go together or are too strong. Too much of a pretty perfume can be overpowering.
Pamper the Princess
Make a spa day or night out of your whirlpool bath. In addition to romantic scents, add plenty of bubbles to the bath. Arrange for a pedicure or manicure. Buy some facial masks and do facials. Don't forget the back rub and the foot rub!
Wash Each Other
Make sure you wash each other! From your head to your toes--having someone else wash you, like a sponge bath, is very intimate. Washing someone's hair for them is relaxing. Dry each other off when you are done too. Bathing together in itself is very intimate and personal, so take it one step further and do the work yourself.
Splurge Away From Home
Find a nice hotel with some great suites with whirlpool tubs. This is an extra treat for your special someone, knowing you spent the money on them, just for the two of you. And what's better--no interruptions! Sybaris Suites has great suites specifically just for couples, some even with swimming pools.