No magic answers. As you are finding, boat financing
is difficult and financing for a privately owned charter
boat even more so.
Generally boats are seen as a high risk loan by banks for many reasons: high depreciation, hard to sell, down market, very portable so easy to "lose" the collateral, high risk of loss or damage, etc, etc, etc.
Pretty much as you noted, if you have enough money
that you don't need a loan to buy a boat then the bank will loan you the money.
If you own a house you can try for a home equity loan.