Do I get a full refund if I cancel?
It depends on the captain's terms and your timing. The deposit is the only money you have paid before the trip, so that is what a refund concerns.
The only money you have paid at that point is the deposit, so that is what is at stake. Read the cancellation terms on your booking for the exact windows. As a general shape across this market, cancelling well ahead usually means the deposit is returned, cancelling close to the date usually does not, and a no-show does not.
If the captain cancels, or the weather makes the trip unsafe, none of that applies to you — see the weather and captain-cancellation articles.