We had a very easy trip both directions, but both days were calm, so maybe we were lucky,
$US25 taxi ride from the international airport. The driver will know which ferry you can make.
It's hard to get the ferry times off the internet. The most accurate times I found were on the Seaside Cabanas website.
I believe it was $US10 each way for adults, a little less for kids ten and under.
Caye Caulker Water taxi was more comfortable the San Pedro Water taxi, both crowded, but CCWT had pads on all the seats, which is a minor point, but did add to the comfort! I wouldn't wait an extra hour just for the pads though.
There's a third water taxi service which lands on the opposite shore of Caye Caulker. I didn't catch it's name, if you need info, you could email your lodging and they will know.