All the CART waterways and rivers including Ribble Link, Trent etc. are perfectly doable on a properly designed series hybrid boat, as is crossing open water areas like the Wash in good weather, because typical power use is quite small and high power (well, not that high, maybe 15kW/20hp maximum for a narrowboat) won't be needed for more than a few hours.
But for a boat to be able to cross the Channel and navigate strong flows on large rivers abroad (as well as travel on the inland canals) where you may need lots of power for long periods -- and long range -- a parallel hybrid with a big diesel (for offshore) and smaller electric motor (for still waters) makes much more sense. If you want something getting on for 100hp for many hours (in a seagoing wideboat) the cost/size/weight of electric motor/batteries for a series hybrid will be huge, and there's still the risk of running out of juice.