The theory is good, but in practice I would rather have a short adequately sized cable going from the alternator
to the starter connection and relying on the very thick battery
cable and high current
The volt drop you are trying to avoid is likely to be greater with a long cable run of lesser gauge direct from alternator
I would suggest that with the split charge system that many boats will have, feeding starter and house batteries, the splitter should be located close to the alternator. Keep cable runs as short as possible with a heavier gauge cable run to the house bank, and the starter battery
feed run to the positive battery connection on the starter motor
as mentioned already.