I already have some sea buoys, but I can't open them for maintenance. That's why I build new ones. I took some plastic boxes with screw threads and glued 3D printed parts on the top and under the bottom. That means they can always be opened if any water is inside or if the battery has to be charged or replaced. Ok, the buoys will be a little bit too large for 1:87 scale, but I didn't find more little plastic boxes.
The first 2 pictures show the unpainted and painted buoys. They all get a little reling sidewards. This is not prototypically but I need the reling for connecting the hook of the crane. The electric equipment will be quite simple: on-off switch, may be a charge connector, LED+ resistor, recharchable LiPo battery and a microprocessor with very low current consumption for flashing the LED.
How much fun you can have with sea buoys shows this little movie taken in Miniatur Wunderland Hamburg:
https://youtu.be/I93qu3JyvwARegardsHarry