A remote tropical island or a live-aboard dive boat often means no shore power for days, which changes the math on what "enough charge" means compared to a normal trip. A phone or dive computer is easy - a laptop, an underwater camera rig with strobes, and a CPAP machine for someone who needs one overnight are a different scale of problem entirely.
A portable power station (a larger sealed battery with AC outlets built in, distinct from a pocket-sized power bank) is rated in watt-hours (Wh), and that number needs to be checked against actual device draw, not just capacity. A CPAP machine typically draws 30-60W continuously overnight, meaning an 8-hour night alone consumes 240-480Wh - a station rated at 300Wh will not make it through a full night for that specific use case, which is a common miscalculation people make when shopping by total capacity alone.
Recharging the station itself on a boat usually means either a generator running periodically or solar panels, and solar input in particular is highly dependent on real sun exposure - a station's advertised solar recharge time assumes clear-sky conditions and an unobstructed panel angle, both of which are optimistic assumptions on a moving boat with rigging casting shade for part of the day.
CairoVolt's portable power station page lists current capacity and real AC output specs for planning trips like this.
