Hi Group 3, good recap of amphibious propulsion technology and nice sketch of your concept hovercraft.
For the next step, I would suggest that for every block of the Functional Analysis, you should list down one or two candidate real-world model of components or items, along with some written justification why you think that particular real-world item is suitable to do its job (eg. ‘Servo motor’ -> Hitec HS-422-> Because it has sufficient torque based on the following calculations (scribble scribble)).
The reason is, you cannot start your actual mechanical design yet without knowing the dimensions and mounting method of the critical components. The PVC pipe or aluminium extrusion frame and 3D printed casings merely ‘wrap around’ those critical parts. By mechanical design, you are expected to draw out a 3D assembly in Inventor or similar CAD software.
Just to be extra safe, I would recommend you find an article or blog by someone else who has successfully build a model RC hovercraft in a DIY manner and gauge if the loads and performance requirements of your project is around that of the prior-art. You can start with past FYP reports that our library (via ELISER) keeps online. Then, you can base your component selection and evaluation off those past projects and upsize/downsize as necessary. Remember those thrust meters you are using for the surface vessel propulsion experiment? They are originally meant for aerial motor+propellers (search for Tahmazo Thrust Meter), and can help you find out if any physical samples of high-speed DC motors+propeller combos give enough thrust for your hovercraft application.