Unpaid Invoice

You cant lose something you never had. 

If theyd like to claim the income they "should" have received, then they can deduct the income that didnt get paid to them...in other words, its a wash.

Re the unpaid receivable.

Cash basis No.

Accrual Basis maybe.

I'll leave the boat to someone with more experience in that area.

There was an All Star here once that stated: You cannot write off what you did not write on.

Think of this: they already have actual expenses from any work they did for the customer (payroll labor and materials). Now they also want to write off an Unpaid Sale? That would be double-dipping. You can't write off lost Profit, unless you first "write ON" the Income. That's why the tax return has one line for Gross Revenue and one line for Allowances/Returns. That would make it a wash:

Gross Revenue not paid minus Write Off same amount = 0.

Yes, people do this when it matters for sales tax reporting, in fact. It's not really a write off for income taxes; it's a Wash.

For the boat trip, you have to read up on the new rules that exclude Entertainment because of the TCJA. You have to determine what they did and if it qualifies or not. Most entertainment does not qualify and never did.