Self flashed fiberglass should extend neal's 6 or 8 inches. The floor of deck itself should be 4 or more inches below interior floors.
Poly sealer over walls will not help. Your going to have to remove some stucco at bottom of wall. Install fiber glass or other flashing up wall 6-8" up behind stucco, bonded to deck floor. 6-8" out. Then do same over stucco to protect it.
You might have to adjust slope of deck. Which could be massive chore.
Please add your location to profile, location plays part in answers that you may not be aware of. For one thing where are you that you get such torrential rains and how often? Is it worthwhile to adjust slope, would a deck cover help, drainage holes, other solutions. Some here may have faced similar problems and be in your area, solved problem or know source of materials or craftsman.
[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]