You will probably need to get it from a specialist supplier but it should be possible to obtain a combined transformer and adaptor. This will 'step-up' the voltage for you as well as enabling you to plug your UK 3 pin plug into the US socket. But in the first instance why not contact Royal Caribbean direct? They might well have a system in place for you to borrow one onboard because these days big companies like that are much more 'disability aware' than they used to be.

However, also check with the company that supplied his Powerchair - you might find that it will still charge up safely but just take longer to do so. The power supply in Cuba is mainly 110V too and I've found that most things will still charge up - though admittedly for smaller appliances than you are needing it for.

SM