imho, the $100 limit is unrealistic. Lowest new 5-way weather station for home use is almost that, and only has 4 AA batteries, no solar panel, no buoy, and no mooring, with a range of ~100 meters, on 433MHz band.
In addition, it will need at least 3 anchors, to help maintain orientation, to identify wind direction and maintain solar orientation, for power.
That is not saying it cannot be done. I'm comparing the basic equipment and adding the physical requirements, as well.
For a Buoy, it will also need navigational markings and lights. For the proposed design, might look at:
Could place the batteries as ballast, inside a pvc pipe, etc.
The spar would need to raise the sensor probably about twice the typical wave height, 2-1/2 would also tend to clear local rogue waves, with the antenna above that. Buoyancy would be determined by the equipment and battery needs.
Beyond that, there is the need to coordinate with the national government, as to the legalities, their requirements, and so forth, before operating a transmitter, as well as placing a uoy, or moorings.