The weather is quite nice here in March (currently right now we are in the low to mid 80's during the day and mid 70's at night). Unless there is a cold or warm front coming in from the states, that would be about what March is like. It won't be hot hot for a ceremony unless you get married mid day. but if you are a few hours before sunset you'll be fine. This time of year we have lovely off shore breezes, in the evening you may even need a light sweater or long sleeves (not cold but when it's windy, it's nice to have if you've been in the sun all day.) Our white sand never gets too hot to walk on, even my kids have no problem walking around in mid day sun on the sand, so there are no worries there.
We only have 2 all-inclusives on island, Beaches Resort and Spa, and the Veranda (we also have a club med, but that is really not a wedding location resort in my opinion and it's a very old dated property). There really isn't an indoor wedding venue here, we just do everything outdoors and it's pretty rare that the weather is bad enough to go indoors for anything. (my house is open air, screens only, no glass windows or a/c). The Regent Palms has an indoor ballroom that could fit in 20-30 guests for a ceremony but that is the only place I can think of. Again, as long as you are not getting married at high noon, you should be very comfortable on the beach or out in a garden setting.
Obviously if you go with Beaches, you'll use their wedding planner and many of the other hotels on island have someone on site. We have 3 independent wedding planners on island. Diane with Island Harmony, Teresa with Tropical Destination Management and Nila Destinations. All have been around for quite some time and know the island and the resorts. Some of them actually work as the resorts designated wedding coordinator if the resort doesn't have someone on staff to do the job. I would initially check out the resorts and see if you can decide on one and then see how you like their on site coordinator. If you feel you need more that what they offer, seek out one of the above.
Hope that helps