I'm using a borrowed Panasonic 14-140mm now, but find it too large, too heavy and too slow (large aperture) to be a great general-pupose lens. Image quality is just OK even though it's probably the best superzoom made for any camera system. And given you'll already own the 12-50mm i wouldn't own a superzoom as well, the 12-50mm is wider at the bottom end and it's sharper and it does macro and it's weather-sealed.
A much more useful lens, IMHO, is the Panasonic 20mm f/1.7. This lens is small, bright and very sharp. If you're just going out to dinner or taking photos indoors, the 20mm f/1.7 is a great lens for available light photography and it's great when you just want your camera to be small. It will compliment your 12-50mm.
At the telephoto end you could add to your 12-50mm with the expensive 75-300mm if you want really amazing reach, or the smaller and much cheaper 40-150mm lens. You can buy both the 40-150mm and 20mm for just $50 more than the 14-150mm, and in my view this would be a much more versatile and useful system for above water photography.
At the ultra wide angle end, your 8mm fisheye will do the trick, use fisheye-hemi software and you can reduce the fisheye effect in photos taken above water. In my view this solution distorts less than using ultra wide angle rectilinear lenses (like the very nice and expensive and much larger 7-14mm).
