To answer your question without opinion:
Ampair aquair 100 is a towed generator for yachts
Ampair Wind, Hydro and Packaged Power specialists +44 (0) 1258 837 266 | Aquair 100
They are designed for charging sail boats at between 4 and 12kts
Max output is little below 6 amps, at around 7kts, (~75 watts)
A typical 55lb thrust electric trolling motor will draw around 50 amps at full speed (600W). Assume you will use close to full power most of the time - average 500W draw - you will have to cruise for around 7 hours to charge the battery enough for 1 hour use.
Some more realistic scenarios: If you are just trolling around, perhaps you need to use the motor for around 20 minutes, in which case a 2.5 hour journey at 7 kts would give you enough charge (probably).
To put it yet another way, a 30 minute cruise to your fishing site (at 7kts max!) would give you just over 4 minutes extra on the electric motor.
Compare this to a 70HP petrol outboard with a 25A alternator. A run of under 2 hours at wide-open-throttle would give you 1 hour on the electric motor, or, a 30 minute journey to your fishing site (at wide open throttle) would give you an extra 18 minutes on the electric motor.
Lots of assumptions here - assume the actual figures could be 20% lower or so with losses due to temperature, gauge of cable, corrosion etc etc
To answer your question with an opinionated answer:
If your engine has a charging coil /circuit you would be crazy not to use that instead of the horribly inefficient, complicated, expensive, faff-tastic water-towed generator. If your engine doesn't have a charging coil it may still be cheaper to fit one rather than buy the water towed generator. Even after this, you're really not adding a lot of extra range to your electric motor and would be restricting yourself to either journeying at 7kts (towed generator) or full throttle (engine alternator) to get any meaningful(!) extra charge into your battery.
If you are going to the trouble of carrying all this extra generator gear, You'd get more range by just carrying another deep cycle battery instead, and it would be cheaper. If your journey from home to the boat ramp takes some time, you might consider a caravan-style hookup to top-up the batteries from your tow vehicle electrics, which might give you the same few extra minutes run-time on the trolling motor.