I'm just throwing this out there for anyone to work on if they chose to. I'm nowhere near an ocean and have plenty of other projects to work on.
This would be a fascinating and challenging project to work on with all kinds of engineering decisions to make in order to produce something at very low cost that could remain at sea for years and still work upon landfall. For instance, would the transparent PET plastic jar (wide mouthed for interior access and easiest type to get as it is the most common food packaging plastic) become sufficiently opaque over years of UV exposure to interfere with the solar cell output to a fatal extent? Would algae accumulation (if any) do the same and would there be any commonly available product that could be used on the exterior of the plastic jar to prevent or reduce it?
The internal components would need to be mounted securely in a way that could withstand potentially years of high-g loads experienced within storm waves, preferably without any penetrations through the wall of the jar. What commonly available adhesives should be used for that and for encapsulation of the ballast (ex., BBs or lead shot). Heat due to solar insulation would be an issue to deal with (use Al foil inside jar as solar shielding above waterline except, of course, in front of solar cell). Hail impacts on the somewhat flexible skyward wall of the jar would need to be considered to avoid damage to the solar cells(s) (ex., recess solar cell(s) from skyward wall). And on and on.
The design should use through-hole components to allow easy kit assembly. Off the top of my head, I don't know of any accelerometer ICs in DIP form for wave motion sensing and landfall detection. I'd suggest using a UV LED as a UV photodiode as shown in my illustration below. Below the waterline, UV would be low. Once landed, the UV detected would be much higher.
UV resistant text would be used for the message and the jar would be marked in some way with something like "Message in a bottle" in several languages. Which languages? English, Spanish, French and Chinese?
Here's the concept:
http://www.slickpic.com/s/zjGMiyNNDOLGjj/_201210261017/photo?view=2369211#2369211