Biscayne Bay is close to a perfect jet ski environment, which is why there are so many of them on it. It is shallow, it is sheltered from ocean swell by the barrier island, it is warm all year, and it has a skyline down one side, a chain of causeway islands through the middle and open water at both ends. On a calm Saturday morning the water between the Venetian Causeway and Star Island is glassy, and you can see why this became one of the biggest personal watercraft markets in the United States.
Before anything else, the paperwork, because this is where visitors get stuck at the dock. Florida requires anyone born on or after 1 January 1988 to hold a Boating Safety Education Identification Card to operate a vessel of ten horsepower or more, and a jet ski is emphatically a vessel. If you were born before that date, a government photo ID is enough. If you were born after it and do not have a card, rental operators can issue a temporary certificate valid for 90 days after a short on-site safety test, which takes about twenty minutes and is the reason your booking says to arrive early. Separately, you must be 14 to operate a personal watercraft in Florida under any circumstances, and 18 to rent one. No adult supervision waives the age-14 floor.
Two more rules that catch people. Every person on a personal watercraft must wear a US Coast Guard approved life jacket, and an inflatable belt does not count for PWC use. And Florida prohibits operating a personal watercraft in the dark: the statute runs from half an hour after sunset to half an hour before sunrise, which in late December means the water closes to jet skis before six in the evening. Sunset tours here are genuinely sunset tours, not evening ones.
The other thing to understand is that "jet ski rental in Miami" usually means a guided tour. Large parts of Biscayne Bay are slow-speed, no-wake or manatee protection zones, the channels carry constant commercial traffic, and free-roaming rentals are restricted or supervised in most of the areas visitors want to be. What you are typically buying is 30, 60 or 90 minutes following a lead rider through a defined route, with open-throttle stretches in the designated areas. That is not a downside, but it does mean a one-hour rental is not one hour of open water, and operators who imply otherwise are overselling.
Beyond the jet ski, this city's water offers more than the marketing suggests. Parasailing runs off Miami Beach, Key Biscayne and the downtown bayfront, and 500 feet above Biscayne Bay is the cheapest aerial view of Miami by a wide margin. The Rickenbacker Causeway has a shallow, breezy stretch that is the region's windsurfing and kiteboarding school. Oleta River State Park in North Miami is the largest urban state park in Florida, with mangrove creeks you can paddle for hours and a mountain bike network alongside. And the mangrove tunnels on Virginia Key are ten minutes from downtown and feel like nowhere near a city of six million people.