Not that it's right, but letters from an attorney can get expensive quick (trust me I bill for them). For three weeks of sea time it may be worth it, but it isn't going to be cheap
to sue on something like this.
Is the captain
a licensed one? If not he may not be aware of how much you need the letter. If he is commercial
, then it's likely you are just at the bottom of his priority list... Get in touch with the boats agent, or commercial
operator and ask for the letter that way. They tend to be much better at shuffling paperwork.