Offer a billion dollar reward on a "no win, no fee" basis, that should even have people out in rowing boats with metal detectors trying to find the thing.
Seriously though, it's all very well having these search vessels out there but they get paid regardless if they find the thing or not thus it is likely that they may merely go through the motions ... Try offering a (very big) incentive for finding the thing whilst allowing them their own discretion of which area of ocean they believe it may be in.