make sure when you launch to the runway you check the contract and everything shows green up to that point?
Fly directly over it and make sure it's flashing on your hud. Also make sure you're going to area 1 first.
You can actually go to area 2, drop one go to area 1, drop and get both notifications, but for verifying it's working just go to area 1.
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EDIT: Here are the craft files for a working bouy and
Working stuff to test with, all vanilla (99% certain).
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The subassembly can be placed on the underside of a wing or in a downward facing cargo bay. I release them at around 120-175m ASL going between 100-130m/s. 120m altitude is somewhat risky to do until you are used to it, however it deploys them fairly quickly onto the water.
The seaplane has notes in the description, I would recommend at least the first upgrade to the SPH for basic action groups so that you can use reverse thrusters and flaps to slow yourself down more easily.
NOTE: for keyboard flight I highly suggest precision controls (caps lock by default) as the craft was designed for joystick flight originally, but you can fly it easily with keyboard as well (reduce the authority limiter if you're having trouble with it.
The seaplane was designed for multiple contracts at once, so has 4 extra seats for passengers.
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Subassembly with decoupler attached (Bouy - Large V3 - Decoupler):
Sea Plane (Seaplane v2.5V - BuoyL):