But you could improve upon the bottle idea by adding a solar panel; small motor; ardino; disk drive; radio. Messages could be stored in this device in bulk and it would eventually reach other destinations. If such devices were cheap to produce you could have thousands of them in the ocean and have them communicate with each other to form an alternative to the Internet. It would be similar to satellite constellations in that infrastructure could be moved to provide connectivity between routes as strategically necessary.
Is anyone building anything like this?
>SSB was created by Dominic Tarr in 2014 as part of experimental development in alternative databases and distributed systems. Tarr lived on a sailboat with unreliable internet connection, and became interested in creating an offline-friendly secure gossip protocol for social networking.
I think that your main problems will be 1 )fouling and 2) getting enough energy from solar panels to overcome wind and ocean currents.
Sea bird poop, algae, barnacles and other kinds of marine life will cover your solar panels. Your propeller will get tangled up in all kinds of crap, there is loads of rubbish in the ocean such as plastic bags.
The amount of energy that you can get from solar panels that are not angled to be perpendicular to the sun is pretty low. It depends on your latitude as well.
Solar power is usually very disappointing when you look at the numbers.
A solar panel that is nominally rated 100watts will only give you 100watts when it is perpendicular to the suns rays on a cloudless day when it is clean.
Over 24hours, including nighttime, you will be lucky to average 5watts from a panel that is nominally rated 100 watts.
Solar panels will produce zero power at night. Are you going to include batteries to power a motor at night or let your craft move with he wind and current at night?
Solar panels can't be twisted or bent or they will crack and not work any more. You need a strong frame.
I think your craft will have to be a reasonable size so are going to need at least a couple of hundred watts for a motor that will overcome even a small water current.
Beyond the horizon, lines of site look into the vast space. Satellites see over the horizon. What you probably want when you want to send better for sending radio signals from one point on the earth's surface to another.
I think when the USDoD looked at this, it went acoustic at extremely low frequencies. But that's only from reading Tom Clancy in thirty five years ago. Good luck.