I used Digicel last month in St. Lucia, they have different data plans from 1 day to a month. It's pretty reasonable I think I paid 25 EC for 500 MB good for 2 weeks. You can roam with the data, but since I was only on St. Liucia I'm not sure what the roaming rates are.
I remember seeing a promotion in the Digicel shop that was "no roaming charges in the Caribbean" but that I'm not sure if that included data and you had to opt in on it.
There is free wifi at the Rodney Bay Marina at Café Ole', Boardwalk Bar, Bread Basket to name a few places, so if you're heading straight to the Grenadines you could get a SVG sim to avoid roaming charges. There is a digicel store right near the marina, just north on the main road, could check into it there to see about roaming.