Having seen way back, a lifeboat have its performance transformed, both speed and run time, buy changing from a lead acid brick to a much lighter is lower voltage and capacity NiCad pack, I can see that a similar improvement in a performance boat will happen changing to lithium.
But the OP did mention a tramp steamer. If there is going to be a lot of internal detail exposed to view, there might be limited space for batteries - I can see a good case for using lithium there, but otherwise, NiMH will do the job nicely. Early NiMH did have a reputation, rightly or wrongly, of discharging itself, probably got from the groups that DM mentioned. I have not noticed that happening, certainly in packs bought in the last two or three years. Then again, I got them from reputable dealers.
Lithium batteries do need proper care. When fitted into consumer devices, they are in an environment controlled by the manufacturer (lots of YouTube footage showing what happens when they get it wrong, laptops, airliners etc). In the case of a model boat, the builder is the one providing the environment, and might not have the expertise of, say, Samsung or Toshiba. Any energy storage system needs proper care, especially anything that delivers DC at potentially very high current.
