It's important that you understand that flare-ups are not as big a problem as you might think. Controlled flare-ups are OK; it's when the fire gets out of control that you have a problem. There are several ways to deal with flare-ups and a spray bottle full of water isn't the best way.
Planning for Flare-Ups
You need to plan on flare-ups and prepare for them. The first step is to reduce the risk of a flare-up by trimming any and all unnecessary fat from the foods you grill.
For the sake of flavor, meats should be left with a little fat on them. This helps prevent meat from drying out while grilling. Removing excess fat also includes oils in marinades or sauces. Marinades should be allowed to sink in and then drain off so marinated meat is not dripping in oil when it hits the grill. By reducing the amount of fat on meats that you grill you reduce the amount of flare-ups.
Have an Escape
Of course, you can't and shouldn't take away all the fat. Foods like chicken with the skin on or a good thick steak or hamburger patties are going to have fat and that fat is going to melt and quite likely catch fire. That's fine. What you need is an escape plan. Unless you are grilling enough food to cover the entire cooking surface of your grill you need to be ready to move meats out of the fire, to another part of the cooking grate. This gives you the ability to keep foods out of the way of a flare-up and prevent burning.
Once you have cleared the area of a flare-up of food, let it burn. Fats exposed to flame and intense heat will burn away quickly. You can also use upper warming racks to hold food temporarily while the flare-ups persist. Like fighting any fire the first step is to get the combustible materials out of the fire.
Watch the Grill
Flare-ups lead to burnt foods only if they go unattended. If you are putting something fatty on the grill you don't have the luxury of rushing in to check the score of the game. In five minutes a grease fire can reduce most anything to ashes, so stay by the grill. Watched food doesn't burn.
Once you have a flare-up going take control of it. By moving meat that is dripping fat into the flare-up you can keep the flare-up in one place. Say you have a dozen chicken thighs going. When a flare-up starts move the chicken out of the way, say to the warming rack. Now drop the chicken pieces into the middle of the flare-up for a few seconds to let the grease on the chicken drain and burn away. Do this with all the pieces, then clear out that area of the grill. Let it burn down while you keep the chicken either to another side or on the warming rack. Worse comes to worse remove food entirely from the grill, let the grease burn off and then resume grilling. If you can't prevent it then you need to be able to control it.
Dousing the Fire
Now if all else fails you can resort to the spray bottle. A water-filled spray bottle is a weapon of last resort. There are several reasons you don't want to use a spray bottle.
First of all, you don’t put water on a grease fire. That's always rule number one. Water and burning grease do not get along. Secondly, while the water will reduce or even put out the fire temporarily it does nothing to get rid of the grease in the grill. So the second that grease heats up again the flare-up will be back. Thirdly, when you spray a grease fire with water the grease explodes (why we don't do this in the first place). That grease splatters over food and, well, let's say that partially burned grease doesn't taste very good. If, however, you end up with an uncontrollable flare-up move the food out of the way, to a plate if you have to, turn off the burners, and spray down the grease to put it out. Now you can start over.
Once you have had that big flare-up it's time to clean out your grill.
Start by letting it heat up and burn away as much grease as you can. Now get in there and clean out all the burnt grease and food from the bottom of your grill. A clean grill really does produce fewer flare-ups.
So remember to plan for your flare-ups. Reduce the risk by cutting back on the fat. Have an escape plan in mind to get food out of the way. And keep a close eye on the foods you cook.