st martin to st john

I don't think anyone runs a ferry service between St Maarten and St John. It's a long way over open water for a ferry.

You can take LIAT from SXM to STT then take a taxi to the ferry in Redhook.

The only ferry service from St. Martin is the one the creeps operate with the poor illegal Haitians & Chinese. You won't have much time on a day trip but you might want to overnight.

http://www.liatairline.com/

You also might get a better deal flying into the BVI's, Beef Island, Tortola. You can get a ferry from Tortola to USVI. (Long taxi ride from airport to ferry.) There is also a private Water Taxi that could pick you up in BVI and take you to St. John. The only way to do it in a day would be a charter flight and private water taxi, and even then, I doubt you could do it. And, we're talking big buck here!