As an FYI, I plan to do at least one day of diving while on a cruise that we will be on in late January. I've researched 2-3 dive ops that are either close to or pick up from the cruise ship in each port we will be in. St. Lucia, with the one op with the crazy cancellation policy, and the high taxi rates I would have to pay to get to the other rule out diving there. Wasn't high on the list anyway.
I can give you my 2 cents (maybe 4 cents?) worth of advice having done a lot of diving off cruises. I've done the southern Caribbean cruise out of San Juan five times in the past 10 years, and we're doing it again the first week of February (so we'll be in St Lucia a week after you are!)
First, booking independent (i.e. NOT the cruise ship excursion) is the way to go.
Second, the policy in your original post is pretty unusual. I found that same policy on the same website that you did while I was recently searching for options for our upcoming trip.
My experience: we dive at every port, every cruise. We only take the cruise to go diving.
At some islands I've varied the dive ops we use, but at St Lucia we have (so far) always used Dive Fair Helen. They are one of the ops that require a "taxi ride", but the dive shop has always arranged this for us. Every time (five times so far...) we have used them they have been 100% solid, no problems whatsoever.
Our itinerary is St Thomas, St Maarten, St Kitts, St Lucia and Barbados. And St Lucia is EASILY the best diving of the group, although each island has its own unique character. Last time we were in St Maarten the reef sharks, including a baby, followed us around like puppies.
I haven't booked our St Lucia diving yet for our upcoming trip, because I was considering using the shop that you pulled your original post from (is it "Scuba Steve's"? I think I have that right...) They offer to pick up with their boat at the harbor where the cruise ship docks. But this harbor is further north, and the prime diving is in the south, near the Pitons.
It seems like a bit of a gamble, i.e. if there's a problem with traffic you might "miss the ship" (or at least that's what the ship people will scare you with.) In all my trips, to these islands as well as cruising to the western caribbean a half dozen times, I have always dived (independent of the ship tour) and never missed the ship.
Dive Fair Helen can take you to the better diving area, and the underwater topography is pretty dramatic... it's the side of a volcano, after all. The Superman drift dive is everything it's reputation promises.
If you do want to dive one of the other islands instead, let me know and I can give you my advice based on having done it all a bunch of times. I've used a couple different dive ops at St Kitts and Barbados, so I can give you pros and cons there. We've always used the same dive op at St Thomas and St Maarten... but things have changed a bit since the hurricane two years ago. Again, I can share with you what we're doing for our trip in February and maybe save you some time researching your options.