Sadly I have never been to the Seychelles so I cannot advise you; but I do have some experience with the Turks and Caicos. I have only done some liveaboard diving trips in TCI - and it was fantastic - dramatic walls, healthy reefs, lots of sea life, plenty of large creatures including turtles, eagle rays, and sharks on almost every dive.
The reason we have chosen liveaboards in TCI is because the general consensus is that the best dive sites are off of French Cay and West Caicos and they are not usually accessible to the land-based boats. I have read that you can expect long boat rides to get to and from the better sites if you are using land-based diving from Provo, and the nearby sites are not the best, but since this is primarily a family vacation with snorkeling and a little diving, that may be fine.
We love Provo and usually spend a week on the island before getting on a liveaboard. I am a big fan of the Cayman Islands but I think that Grace Bay Beach on Provo is even better than Seven Mile Beach on Grand Cayman - the powdery white sand and the turquoise water are incredibly beautiful.
Provo is a nice mixture of old-fashioned island quaintness and modern tourist location so you get the best of both worlds. It is very expensive, the US dollar is the unit of currency but it doesn't buy much. We like to stay in an apartment or condo (like the Royal West Indies) with access to a full kitchen and a washer and dryer. We prepare some simple breakfast and lunch meals and may go out for dinner. There are some very lovely restaurants on the island. We like Opus the best. English is the official language. The cruise ships dock on Grand Turk - at the cruise ship port - and not Provo so that is nice.
I am not a fan of big mega resorts (but I've only tried one - a Club Med - and didn't care for it.) But you might try looking into the all-inclusive Beaches Resort on Provo because it offers water sports and activities for the entire family. The resort also offers scuba at no additional cost. I have read both negative and positive things about the Beaches scuba offerings so you should research it on this forum. But again, if you are primarily on a family vacation but would like to get in some dives, it might be a good option for you.
In March the water temp on the reef will be a little chilly, probably in the mid-70s F. That's not bad but if you like it warmer then that's something to consider. You should drink bottled water because they have had some outbreaks of Norovirus (unless your resort provides filtered, treated water?) I haven't heard much about Dengue or Chikungunya on TCI but it is always wise to use insect repellent in the Caribbean.
Good luck and I hope you have a great trip wherever you go!
i'm trying to decide between going to T&C or the Seychelles in the end of march 2016. It you have been in either place, please tell me pro's and cons - particularly your experiences with diving. I will be going with my family, and two of us are divers, the rest are not - they do love snorkeling and other water-related activities though. 
