Well, some interesting pondering it is!
The German balanced antenna tuner is very interesting, indeed, but not for a boat I think. Could be great in a land setup, though, where you want to run more than about 150 watts PEP output....the limit of many automatic tuners. While the $700 price
is a bit steep, the unit seems to be well designed and constructed, and it's considerably less than you'd pay for a good high-power end-fed auto tuner like the SG-235.
I think the limitations for boat use are of two types:
1. to get an all-band balanced dipole antenna, you'd need a pretty long one...probably 100 feet or more. On most small boats, there simply wouldn't be enough room; and
2. the radiation pattern would vary greatly by band, and likely would do so in an unpredictable way.
I don't know if you were thinking about a vertical or sloping orientation, or an inverted vee, or what. Either way, my guess is that the two considerations above would be seriously limiting aboard a small boat.
Could be great fun on a large ship, though, or a home QTH :-)
I've been trying for many years to come up with a multi-band solution better than the traditional backstay, and which doesn't require a counterpoise
Keep up the pondering. Maybe you'll have a breakthrough.