The propagation charts
are accurate for propagation
. But of course they don't indicate noise
levels at either end, the distant station being in use but unheard, the channel being in use but unheard, unexpected variations in the atmosphere, etc. I regularly use the propagation chart wavelengths that are less than optimum, often with excellent results. You were correct to suggest other wavelengths.
Can't speak to the 1.5kHz split...Sailmail takes care of this with no interaction or configuration actions on part of the user, thankfully.
Other boats nearby are no help at all as they can absorb much power while generating plenty of noise
. Plus the ground plane will be less as the boat is out of the water