It's Biscayne National Park, not Key Biscayne. Key Biscayne is a largely
residential island about 20 miles north of the park entrance. The entrance
is closer to Homestead than to Key Biscayne.  Access is from Convoy Point,
near the Turkey Point Nuclear power plant. There's a fairly long boat ride.
All the good diving is outside of the fringing islands. The islands
themselves are around 5 miles off shore at that point. I'm not sure where
the park boats cross the islands. There are a lot of shallows out there and
only a couple of channels suitable for vessels the size of most dive boats.

Diving in the park can be pretty good. Because the entrance lies about half
way between Miami and Key Largo, it is frequently overlooked by those
traveling to the area for diving or snorkeling. Some of the reefs in the
park are not as heavily impacted by divers as those to the north and south.
That doesn't mean, however, that they're pristine. The local boating/diving
community is well aware of the sites and has been visiting them for as long
as I've been alive. Fowey Reef used to be one of the most dived sites in the
area, largely because it's easy to find and reach from Miami. There's a
large light tower at the reef. Sites further south are less often visited.

Park waters are easy for me to reach even in my smallest boat. I've never
used the park dive operation and know nothing about them except that they
used to be a private franchise rather than a government run service. I
presume they still are. I would expect the typical park type of cattle boat
operation catering to both divers and snorkelers. Beyond that, I can't tell
you much. There are other dive operations in the area. Some visit the more
northern portions of the park from Key Biscayne and Bayside (Miami) marinas.
I'm sure there are some leaving out of Dinner Key and Black Point marinas
progressively further south.

Lee

It has been a few years since I dove from the park dive service. It
was good. Boat overloaded with safety devices, ladders, life jackets,
water, you name it. Sturdy boat. Knowledgable crew. Easy diving.
Competitive pricing. Good rental gear