You have several options...
* Dexter Charters over in Sandy Ground on the French Side
* Dougie at Taylor Made in Simpson Bay
* Rudy's Deep Sea Fishing Charters in Simpson Bay
* Anthony in Simpson Bay runs a smaller center console boat (http//papa-smurf-charters.business)
* In Phillipsburg, there is a mob called SXM Exterme Fishing - they advertise on Facebook but I can't recall their phone/website.
* There was a guy working as inshore fishing guide (Tarpon, Shnook, etc) but I'm not sure he is operating anymore

Lastly, I also run a Fishing Charter - we are Coconut Reef Charters, based in Cole Bay.

...I will leave it to you to find which one suits you best.

In March, it is still prime Wahoo season. Average size around here is 35 - 60lbs. Always a chance for a Mahi-Mahi but prime season is typically Summer to late Fall. Same goes for Tuna. Bottom fishing is always possible, especially on electric reels, deep dropping - Red Sanpper: Queens (the best, IMHO), Yellow Eye and Silkies. In shallow waters, the local favorites are Butterfish and Old Wive (Triggerfish), to name a few. If you're after billfish, you're options may be a bit limited but they are here, typically in the summer months to late fall but I have hit a White Marlin and a Sailfish in the same week in Feb/Mar.

Lastly, I will say, we are not like Cabo, Costa Rica or certain places up in the US Gulfstream. Don't expect to go home with a freezer full of fresh fish. Still - we do get some good fish around here and pretty regurlary.

Best of luck.
Craig @ Coconut Reef Charters
Cole Bay, St Maarten (Dutch Side).