You don't need a swimming pool to have a water-themed birthday party, although you do need some outdoor space, a bit of warm weather and probably some towels. You can make your party guests feel refreshed with some cool games and activities that will leave them thinking they have visited a water park.
Break the ice with a water race that will get your little party guests working in teams. Place two buckets of water at one end of the backyard and two empty buckets at the other end. Split the children into two groups and have them race to transfer all the water from one bucket to the other using a cup. One member of each team should go at a time, then pass the cup to the next member and so on. If you want to make it even more difficult, get them to transfer the water using a sponge.
Turn the classic game of musical statues into something a bit soggier. When the music stops, the guests have to freeze and try to stay still whilst being showered with water. Adults can run around squirting them with water pistols or switch a sprinkler on and off. Make sure your music source is safely away from any water.
Fill balloons with water and just watch your guests burst with joy as the balloons explode all over them. Challenge them to throw and catch the balloons to each other or on their own. Get them to hold the balloons on their heads and navigate around a simple obstacle course without bursting the balloons. Or, use water balloons for a game of tag, where the person catching has to throw the water bomb at the other children to tag them. You could use water balloons instead of a present in pass-the-parcel: when the music stops, the kids have to burst the balloon on themselves before winning a candy, instead of unwrapping a parcel.
Place buckets of water placed around the backyard, then give all the children a paintbrush -- as big as you can find -- and set them loose. They are likely to forget they are only painting with water and will be overjoyed at being allowed to "paint" wherever they like. Encourage them to draw pictures on the fences or patio and see if they can finish their creations before the first part starts to dry.