Easter is early enough in March that most of the spring break business will probably be tailing off... anyone here got kids that can chime in? If spring breaks are happening then, all bets are off as far as short notice bookings. During spring break, advance booking is really worthwhile. I'd say if you aren't flexible with your schedule, at least a week or two advance would be helpful. If you wait 'til a day or two ahead of time you may be making several phone calls but you should probably be able to find a spot.
The mantas are very consistant off Keauhou, usually 1-3 nightly, and several of us will go down there if it's not happening at Garden Eel Cove and the conditions aren't horrible. If the mantas are at Gardern Eel Cove, that's a much preferred dive site as the manta numbers are generally larger, there's more moorings, it's more protected from swells and there's nice reef to look at at the tail end of the dive. As long as you book with someone who's willing to do either site, you've got a pretty good shot at seeing the mantas.