Travel
Because they fill different parts of the night. A boat party gives you open air, the bay and a relaxed start with your group. A club like LIV gives you the late night energy, big DJs and the Miami Beach scene. Together, you get the full Miami experience in one evening.
It also solves a common problem: what to do before the club opens. Most big Miami clubs don't really get going until late, so the early evening is free. Spending it on the water is a lot more memorable than waiting around in a hotel room.
A few more reasons it works so well: