I would personally run this by an Engineer. A dock behind a house, no matter how long was probably designed with the intent of lighter pleasure craft and was probably built to those specs. A boat you are talking about I think would fall under commercial spec's (we've only had a few warfs come thru the office) and are built heavier and have the piles embedded deeper.
8" diameter, 16 foot centers, 6-8 foot embedment vs. 20" diameter, 8 foot centers, 15 foot embedment. One of those docks will hold a tugboat in a hurricane, the other will come up with a spring tide. . .
Just an example, don't quote my numbers please