We can't estimate how long it would take for seals to break down.
In terms of searching for missing containers, I assume you would be looking for containers that have dangerous goods or pollutants or things that could damage the marine ecosystem first and foremost. Would you bypass the containers you found that aren't an imminent threat to the ecosystem and continue looking for ones that are a real threat, and then go back to get the ones that are less volatile at a later period?