Your charter company recommended travel insurance and you chose not to buy it. That puts you in a weak position. It's possible it wouldn't have covered, unless you got sick or the government prevents you from traveling.

Your best course of action is to be as nice and reasonable as possible. Remind them you are a loyal customer and would like to work with them. It's unlikely they will find anyone to fill your slot, and they may still owe the yacht owner money, depending on their contract. Maybe ask if they would give you a credit toward low season charter.