We did the tour just last week and booked 2 days before through the hotel activities desk. Admittedly it wasn't high season but the boat didn't seem full either. It was $185 US each. They pick you up at the hotel and bus you to the Rodney Bay marina. After leaving there it is approximately 90 minutes to Fort De France. Once in Martinique they drop off a crew member with all of the passports while the passengers get a quick tour of the harbour and then 30 minutes at a small duty free shop in the cruise port.
After that it is time to reboard and sail the short distance to the town centre dock, at which point there is about 2 1/2 hours allowed ashore. You can go off independently or take a short 45 minute walking tour before going on your own.
After reboarding the boat they serve lunch before sailing to Anse Noir for an hour of snorkelling before the journey back to Rodney Bay .
All food and liquor is included, although the food was nothing special, being hot dogs, burgers, grilled chicken and grilled fish, all catered to a back yard BBQ standard. It was adequate but I think I would rather have had more time ashore and have eaten there first. Having said that the flambeed bananas for dessert were delicious.
Euros are required in Martinique and you won't get by with USD except in the duty free store.. They do stop at an ATM on the walking tour to allow those who want to to withdraw money. We got a small amount before we left St Lucia, although that required a 40 minute line up on a Monday morning. You might be better getting them at home first.. You can also use CC if you want to make any larger purchases.
Overall we were glad we took the trip but you won't have time to see much of Martinique outside of the centre of the capital. I suspect that isn't the best part of the island and it would be like judging St Lucia if all you got to see was downtown Castries.
Your passport won't be stamped either if that's important to you.
Finally although the trip is on Thursdays I believe that they do a Saturday shopping trip once a month too but with a different itinary. You might be there for that as I believe that it isthe first Saturday of each month.