The typical charter boat has only a few house bank batteries, and usually these are in bad condition and won't store even close to their rated capacity. Let's assume 3x100Ah battery. Realistically that gives you less than 150Ah capacity to work with (only charged to 90% due to lead-acid and discharge only to 50%).

The CPAP with 90Watts will use 75A over a 10-hour sleep period. That leaves only 75A and with a fridge, freezer and anchor light that is cutting it very close. And if anyone else on board is using or charging smartphones then you'll most likely discharge the batteries before dawn. Do CPAP devices have a low voltage/power acoustic warning?

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