Honestly - sounds like an AMAZING project. I personally develop autonomous USV systems and am trying to make an ocean-ready version! Check out my project here: https://rlxamerica.wixsite.com/projectorrca
BTW ^ that’s not just shameless self advertising, that’s to show you what I did with Ardupilot and arduboat and the website has many resources on how I did things.
Some other things to look at - solar power the easiest way to do this is use a 12v lead-acid batter (like a car battery). These type of batteries can be hooked up into a solar power system and then directly to the “power-in” on your craft.
You’ll need these:
(1) Solar Charge Controller
(2) Lead-Acid Battery
(3) Solar Panel
Both the solar panel and the battery hook up to the charge controller, then you can wire the DC outputs on the charge controller straight to the ESC of your boat.
Few more tips for you - you’re going to need a REALLY strong hull and a relatively large boat. And I’m talking large like a 12 foot kayak would be the SMALLEST I would go. Also our craft would need to be made of fiberglass or some high-strength material that could survive ocean waves, swells, etc. Propulsion system wise, I would recommend multiple Blue Robotics T200 systems and their respective ESCs - I use the T200 on my craft and have never been disappointing.
I wish you the best of luck - again I have some experience with these things so do let me know if there is anything I can do to help out! Interesting project, ambitious, I love it!