It would help to know if you're looking for something more of an excursion package or if you simply just want to travel to St. Croix. If all you want to do is just travel to St. Croix I recommend you use either the VI SeaTrans Ferry (they only operate on Friday, Saturday, Sunday and Monday; $90 round trip) or take a plane (Seaborne or Cape Air).
Honestly I'm not sure any of the local charters sail between St. Thomas and St. Croix. If you want to negotiate though I've linked some sites you can check out in my source.
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