Looking at your first link (vesselfinder.com), if you read the "about" link at the bottom, it says it is using AIS terrestrial stations to track ships. AIS (Automatic Identification System) uses VHF to transmit vessel location, identification, etc. If the website is only using terrestrial stations, it won't show any ships beyond VHF range from shore. For big ships with high antennas that might be only 100 miles or so from shore. If you look at the map, it only shows vessels near shore and near various islands. Zoom in and you will see lots of ships at sea coming and going near big ports.

A more recent development called "S-AIS" uses satellites to receive data from ships, but it doesn't appear that this website is using that. And I don't know if all vessels are using S-AIS either.

Bottom line is that the absence of vessels at sea is due to an absence of data, not an absence of ships at sea. Trade may be declining somewhat, perhaps related to China's economic downturn, but I'm not too worried that "Store shelves will be bare."