I organized the private mixology groups for my SWDAS cruise last month. We ended up having 4 groups (120 people total I think) so
DCL gave me 3 slots on sea days and 1 slot on a port day. It was ST Thomas day at 4pm and all ashore was 4pm that day as well. If you only have 1 or 2 groups then I'm sure they will try to do it all on sea days, but that was my recent experience.