Some people see going on a water slide as risky behaviour so it may be that you are engaging some risks in your waking life.
Water can signify emotions so it may be that you feel you are in a fragile place. Perhaps you are being carried away by your feelings and focusing on the negatives rather than the positives in your life.
Another interpretation is that you are simply going with the flow and not objecting to or resisting anything even if you don't agree with it or you want to create some waves.
Feelings associated with a water slide are often a combination of fear and excitement. Perhaps this the place you're in within your life right now. Maybe you are going through some changes that fill you with both emotions- it could be a new job, a new investment or a new relationship. Maybe you have given yourself a new challenge and it's scary but you have a good feeling that it will all be ok.
Going down feet first means that you might be going through a period of uncertainly but you will land on your feet when it's all over.
Another interpretation is that you need more fun and adventure in your life and have neglected this aspect of yourself. Maybe you need to be more open to new experiences and not so cautious all the time.