We sailed a one-way from St. Lucia to Grenada last February. Was an interesting trip and quite different than our other sailing vacations. While I can't comment on your provisioning or pre-boarding questions, I will offer advice on a few nice beaches.

First as others have mentioned, Salt Whistle is a protected bay with a nice sandy beach. We spent a day there enjoying the restaurant/bar and my son and I hopped over to the bay to get some kiteboarding in. It was a nice respite after sailing in pretty squally conditions the past few days and we felt quite safe there - at least relative to some of our prior nights stops.

We did hit the Tobago Cays, however, we had cloudy skies with high wind and rain squalls coming though so we were not able to fully enjoy them. Still worth a visit for us and we did have a great lobster dinner delivered to the boat. I can see the appeal of the Cays in more settled conditions and would definitely return on future trips.

Probably our favorite beach of the trip was Sandy Island off the west shore of Carriacou. Though just a sliver of an island and no infrastructure, it had a great sandy beach and was the perfect place to relax. We probably should have spent the night there but we ended up going around the corner to Tyrell Bay which didn't do much for us.

We stopped at the underwater sculpture park and it was ok. Not quite a cool as I was hoping, partly because visibility was poor, but still glad we did it. Since you will be going right by it I would make it a stop if it works into your schedule. Not sure I'd do it if you are going to lose a day somewhere else though.

On our trip we pretty much had 20-25 knots every day with swells in the 6-8'+ range between islands. I think those are pretty typical conditions for December/January/February. Was not a problem for us as we were sailing on a beam to broad reach at all times (which made for pretty spectacular sailing) but it would have been a bumpy ride upwind.