Will be in St Thomas/St John over Christmas (fingers crossed) and might spend a day or 2 in St Croix - the one USVI/BVI we haven't been to. With it? Ferry from St Thomas?
The last few times I went to the USVI, there wasn't a ferry available to take to St Croix from any of the islands. Only way to get there was with a puddle jumper plane. I personally would skip St Croix, don't think it's as beautiful as the other islands.