so couple of thoughts.
Buoys do three important things.
One, they allow people to visualize from the surface where different objects are under the surface. For a nav course it allows the students to practice shooting bearings.
Two, for open water type classes, it allows instructors to kick out on the surface and have students practice a controlled descent to the platforms. Important for their first few descents.
Three, allows for controlled ascents for scenarios like OW instances above, or for divers that have to make an ascent in the middle of said course for whatever reason.
Buoys should be large enough that boats are well aware because there may be divers congregated at them and if you have a class scenario there is a risk that too many markers will be very difficult to manage.
Map below is where we train. You can blow it up to a bigger size. Here is our navigation course for our open water students.
Entry 2 -> south platform
south platform -> bus
bus -> plane
plane -> VW
VW -> north platform
north platform -> where that 306 bearing crosses the blue line which is a contour line
follow that contour line back to the VW
VW -> entry 2
Now, this is for our brand new open water divers, this is their Dive #3 and #4. All compass navigation except for the south platform to the bus, and the contour line coming back to the VW. Lots of compass navigation, all based on waypoints. You need waypoints because these divers have no idea how far they're actually going because it is difficult to gauge and count kick cycles etc. You can use waypoints as simple as old motorcycles or cars you can get on the cheap etc and have one platform for teaching, if it's busy put the second one in. It should be cheaper to get a small car or 4wheeler that doesn't work and sink it for waypoints. Gives the students something to look forward to at the end of the waypoints.