Your best strategy for losing water (if you don't want to take illicit drugs) is Sodium loading.
Start lowing your sodium about 10-15 days out. Don't deplete your levels too quickly - this is to prevent too much aldosterone getting released (Aldosterone is a steriod that causes the body to retain fluid).
*Also increase your potassium a little more than normal, this helps.
15 Days Out - Begin sodium loading. Salt or season all meals and eat foods high in sodium.
10 Days Out - Reduce sodium to 4000 mg per day.
5 Days Out - Reduce sodium to 2000 mg per day.
2 Days Out - Reduce sodium to 1500 mg.
1 Day Out - Reduce sodium to 1000 mg.
Drink 2-3 gallons of water the days leading up to the beach, then cut it by 30% a day before, then drink enough water to quench thirst the day... but nothing more. Don't forget, muscle tissue is mostly water, if you cut your water too much you will lose alot of fullness.
5 days before, increase you carb intake higher than normal (150g+) for 2 days, followed by 3 days none (0g).
On the day:
2 hours before the beach: Ingest a high carb/fat/sodium meal containing approximately 75-90 g of carbs, 20-30 g fat, and 2000 mg sodium. This meal will really help you to fill out.
(By sodium depleting for several days prior you will have increased your body's ability to store more sodium. Since your cells have been deprived of sodium for several days they will be more likely to retain sodium (and the water that goes along with it) intracellularly and store less of it subcutaneously. Couple this with the increase glycogen retention inside the cell and you will fill out.)
Hope this helps