The short answer is NO you cannot expect a reduction in your cruise price due to VAT change.
The longer answer is :
All RCCL cruises call on ports of more than one country and are therefore
classed as "international passenger transport" for VAT purposes.
International passenger transport is zero rated for VAT purposes.
This means that no VAT is levied on the ticket price of the cruise
Please note that VAT may be applied to onboard goods / services dependent
on itinerary and home port.
I think the vat issue all depends on how it appears on your bill/invoice. If the price of the holiday says it's inclusive of VAT then the price would potentially remain the same unless the travel company you book with decide to offer a refund and pass the reduction in vat to it's customers...if it's inclusive of vat then they dont necessarily have to pass on the savings to you. If on the other hand you have booked a holiday that clearly states on your bill is cost of holiday + vat and you haven't paid your invoice then the VAT should be the 15%. Remember it’s a 2.1% not 2.5% price cut so not that great a saving. Imagine a holiday at £100 before VAT. Add the tax and it now costs £115, where it used to cost £117.50. This means you save £2.50 per £117.50 (not per £100) a cut of 2.13%.
Cruises are classed as 'transport' and not subject to VAT. Do not put the idea in the Chancellor's head!