If you book your entire flight straight through with one airline, (say USAir for example), Winair uses the same baggage allowances as USAir. If you book your flight to St Maarten (SXM) and then book Winair to Saba seperately you could be charged for the extra baggage. I usually use Travelocity, punch in the airport codes, the dates I want and away I go.
Even in the event you are charged for extra baggage, they still take it but sometimes it might be on another flight then the one you are if they can't fit it on. The plane only holds 19 people, the Saba runway is only 1200 feet long, you'll understand when you see it. If you are checking in very close to your Winair departure time they might already have all the baggage they can take on your flight so they bring it the next flight to the Island. It really isn't a problem, I've had more trouble with USAir losing my bags then I have ever had with Winair. If you get to the plane and your carry on looks a little big they will put it in the hold for the flight or make you leave it at the back of the plane under the seats.
As for the boats. If it's rough you'll wish you were on the plane. One Ferry is a Cat called " The Edge" and the other is a V hull called "The Dawn II". "The Edge" isn't too bad but "The Dawn II" can be very nasty and sometimes smelly and you sit below deck, not too many windows either. Both ferries only make the trip to Saba on certain days of the week and only once per day. 'The Dawn II" is way slower then "The Edge" and depending on water conditions it can sometimes take 2+ hours on "The Edge".
Here's a link to "The Dawn II" (V Hull)
Saba C-Transport, N.V. DAWN II - The Saba Ferry traveling to Saba & Sint Maarten three times a week.
Here's a link to "The Edge" (CAT)
Aqua Mania - ...just add water! - HOME
Look under bookings for trips to Saba by The Edge.
Here's a link to the Saba Tourism site
Saba - The Official Website of the Saba Tourist Bureau
I've been to Saba a number of times and I'm going again this August, I've used Winair every time and would recommend it. Yes if the weather is bad it can be a pain but so can the Ferry because it doesn't run to Saba everyday, when it does it usually arrives around noon and leaves around 3pm. So what if you get stuck on the Island just enjoy the extra day, the world will still be there when you get back to it. (You might not want to leave once you're there anyway!)
Not sure about this but I was told when you book your flights right through with the one airline if you happen to miss your flight due to weather it's not as much of a hassle when you finally make it off the Island, the airline will just put you on the next plane that has room. We went for a month last year and when we got to St Maarten USAir had lost 3 of our bags. We had to wait, fill out paper work and ended up missing our Winair flight to Saba and spent the night in SXM (St Maarten), it was the last flight of the day. The next day we went to the airport got new tickets from Winair and were on the first flight to Saba, no problem, no extra charge. That trip we even booked Winair seperately from the rest of our flights. When USAir found our bags Winair brought them over as soon as they could.
I would also highly recommend Saba Divers, but all the dive shops are good in their own way. Saba Deep has the smallest boat, Sea Saba has the largest boat, Saba Divers is just the right size. Saba Divers is owned by "Scout's Place" and the crew that runs the shop is top notch and loads of fun. Also if you dive nitrox there's no extra charge if go with Saba Divers.
Hope this helps you out.