Could a landing gear jettison mechanism be practical when ditching?
According to at least one source (based on NTSB data) there were 179 registered ditchings of GA aircraft over a period of 8 years, or an average of approx 22 per year. The trend however were going downwards from 30 in the mid-80s to 12-15 ditchings in the mid-90s when the study ended. 

In all 22 fatalities were registered, with an 88% overall survival rate but a 92% egress rate. The egress rate is what is important for your question. Discard the long-range ocean-ferry flights and this number rises to 95%. 

It's worth mentioning that there numbers are for registered ditchings. Unregistered ditchings are more likely to be successful than not, so the real survival rate is likely to be higher. 

According to the report:


  "Where and when you ditch matters more than what you ditch. Examining the fatal accidents, we found that two-thirds of the 22 occurred during the winter in cold or temperate climates and 12 percent are what we call "blue water" ditchings in the open Atlantic or Pacific, done by ferry pilots on extraordinary missions in light singles or twins, or fish spotters operating far from shore."


If you want to have a landing gear that can be jettisoned, chances are that it would add quite a bit of complexity and maintenance to address a non-problem. 9 of the 22 fatalities involved an aircraft with retractable landing gear, but the study makes it clear that the status of the landing gear was unknown, so we don't know if or how this affected the statistics. 

The problem isn't ditching with or without a landing gear, it's the fact that you're ditching in the first place. And when it happens it most likely does so without you having appropriate survival gear.