Tourists joins in with everyone, and foreigners are frequently extra popular targets for water splashing. Songkran can be a good opportunity to get to know locals, especially if you are travelling alone. The Thai are very including, and you can easily join a party anywhere or one of the many pick-up's that drive around with a barel of water and a joyful crowd in the back. Contributing a block of ice for the water or some drinks will make you popular.
The idea with ice in the water is to make people really feel that they have been splashed, even when they are already soaked through.