You have to use HGVC's affiliated cruise booker... Cruises Only, accessed thru the member's website under My Club - ClubPartner Perks. Here's how it works:
"Enter the amount of ClubPoints you would like to apply to your cruise and click 'Submit.' Then choose your preferred itinerary and select 'Search' to view the results of your search. If the cost of the cruise you have selected exceeds the amount of ClubPoints you have entered, you will also be shown the additional cost in dollars needed to purchase your preferred cruise."
A few folks here find it to be a pleasant way to enjoy left-over points. IMHO, one should use points to book a cruise only if you have no other use for them, 'cuz the points per $1.00 reduction in price is, IMHO, not a good value compared to other uses, including HHonors. But then, YMMV.