We are arriving in Tortola in early March just for the day. What beach can we take a cab to where we can rent surfboards and find descent surf?

There are a few to choose from. Check out this page to see them all:
http://www.destinationcaribbean.net/...p;parentid=231

Little Apple Bay and Josiah's Bay are the best surfing spots (IMO) on Tortola. I know you can rent boards on Josiah's but I can't recall where you can rent them in LAB.

boards can be rented in Cane Garden Bay, not as lively waves there unles there's a swell

I should have listened to your advice, or researched more carefully. We went to Cane Garden Bay and there were "no" waves "at all." We were extremely disappointed. I've stayed on St. John which has calm waters but heard Tortola can be a descent surfing island. One other cruiser went to "Long Beach" (I believe the name is right), and caught a few waves. Stay away from Cane Garden Bay if cruise ships are at port. They charge for chairs to sit in, and the bar at the right end of the beach blares music (so you can't hear the ocean). Very touristy for the BVI, not as peaceful as any beach on St. John.