After seeing the photos and location this was a high risk endeavor.
I don't think you would be liable.
First, most states have reward provisions that preclude the reward offer from liability so long as the nature of the activity was not illegal. This maybe covered in the scope of the protections provided by the Good Samaritan laws and the provisions of incentive or bounty based offers in your state. Second, he is operating as an independent contractor, as there is financial gain, and therefore assumes his own liability. He does not have an employment contract stipulating when and how he does the job. Third, if he exercised poor judgement, such as operating in windy conditions, in a reckless manner or a manner that incurred undue risk liking flying low over a body of water in windy conditions you would not be accountable. Four, he is trained in search and rescue and therefore inherently had knowledge of proper procedure, the risk and potential loss associated with such an operator. Fifth, he was not operating as a volunteer firefight at the time or on behalf of the fire department. This is relevant to demonstrate he was acting on his own accord and engaged in seeking personal rewards which government employees and volunteers are receiving while conducting government business. If he stated or inferred in some type of documentable communication that he was operating as a volunteer firefight he would be ineligible for the reward.
As per giving him $150 for waterlogged recovered drone which you offered a reward of $300 that would more than likely be a breach of contract. In essence, you offered a reward for recovery. The condition of the item is not subject the reward amount when he engaged in the search nor is it contingency based on the value of the recovered item. Therefore, you entered into a promissory agreement and should pay him the full $300 were he to recover your drone.
In your initial post you questioned his motives and in doing so asking for proof of his loss before compensating him in "good faith" for his loss would be reasonable.
Compensating him for a portion of his provable loss would be a kind thing to do but more than likely not legally required. It could even cause you to incur liability and it could be perceived as a casualty condition of the agreement. Even if he lost his drone searching for yours there is always the aforementioned arguments. If you felt morally or personal obligation to offset his loss a gift card would be a reasonable way to help him. A gift card, by it's name, is a gift and would be difficult to construed as financial instrument associated with a promissory agreement.
This is probably overkill and nothing will come of it. It sounds like someone combining learned skills, a training exercise, poor judgment and seeking a reward who had an unfortunate
