See the post directly above your post, where I refer to them as "the very best water generators." And I will quibble with your description of them as 'reliable'. At least three W&S hydrogenerators have failed in this edition of the Vendee, causing (indirectly) one sailor to retire from the race and the others to expend much effort on repairs. They probably all hit debris, but that fact touches on a basic issue with hydrogeneration. The units are all vulnerable to impacts & entanglement. Or shark attack, in the case of tow generators
According to Beth Leonard, Water generators rated second (after watermakers) in the SSCA's "Products most prone to breakdowns" survey -- 28% per annum. Small sample size, but they do have a poor reputation for reliability. As do wind turbines. They work hard in snotty conditions.
A combined troller/generator as proposed by the OP would help mitigate the cost by serving two purposes; OTOH, it would suck to have thieves steal both your outboard & power generating device, or to hit a basking sunfish & lose not only your daily watts, but your water taxi too.