Likely someone has already posted something about this subject, but just in case they haven't, have a look: "These fully submersible watercraft use their forward momentum and the downward lift of their wings to literally fly below the water's surface. This radical departure from the typical method of sinking below the surface allows them to achieve an unparalleled level of freestyle performance" http://www.innespace.com/
I have read about designs based on dolphins. This is like one I saw on the tv show "SeaQuest" I think it would be a nice toy for some big Yacht
Another suitably dolphin-inspired vessel...?! On a more serious note, I liked to think that most of the tuna that we consume nowadays was "dolphin-friendly". And that everyone, from the fishermen, through the food companies down to the consumer were finally behaving responsibly. And then I read this article in the current issue of the Economist. All I can suggest is that we humans, for all our inventiveness and apparent ability to find solutions to problems, are as close to truly solving the problems as Jon Stewart is with his "Daily Show": PS. Whilst the "dolphin-inspired" water-toy Brian shows appears to be a great idea, it has similarly not been "fully thought through". Anything like it should only be used in a closed and controlled-environment in my opinion, preferably developed as a ride in some amusement waterpark and guided by rails. Certainly not for use in open waters, where it would appear to pose an important hazard to its occupants in addition to any others sharing the same waters (whether they be other marine mammals, fish or humans)... PPS. Sorry for the thread-drift...?!