Not sure I can offer much advice, but I can relate. Just booked an Antigua > St. Martin charter for next June for my family of five and am currently hunting for airfare. It's not spring break, but it is the beginning of summer so similar situation. Right now we are looking at around $800-1000 each and that is with less than ideal routing - i.e. overnight and multiple layovers. I got excited when I thought I found fairly direct first class tickets for the return leg from SXM to PDX for just a bit more than the coach fares, but of course they weren't available when I tried to book them. Haven't had any better luck looking for the PDX-ANU leg either. JetBlue is still a month away from releasing their June tickets - they sometimes have good prices on one-way flights. Our charter company is also flexible as to moving the charter dates around a bit to accommodate our flight schedule. It almost looks like it might be cheaper to fly into and/or out of San Juan and book tickets on Liat to the islands.

In the past I have always tried to book the boat and tickets together for the most piece of mind. Most charter companies will hold a boat for you for around a week, which I find offers plenty of time to book airfare. Also, research one-way tickets. I have found that it can sometimes be cheaper and more convenient to fly different airlines each way. In theory waiting until the optimal time to book tickets (2-3 months out) will get you the best prices - until it doesn't. And when it doesn't it can cost you both in terms of price and convenience. I plan on buying tickets as soon as I see some prices that sound reasonable to me. And then I won't look at ticket prices anymore, because I can pretty much guarantee that they will go down the day after I buy them.