1. It means to engage in sexual activity, not just intercourse, but also masturbation, mutual masturbation, oral sex, and stripping/ dirty dancing.
2. If you are sexually active, you jump in the pool. If you DIVE in the pool, you enjoy sex and do it often. If you're a virgin or a born again virgin, you do NOT jump in the pool.
3. Can also refer to an orgy, like if someone asks if you've been to a pool party recently.