Re: Flying US and British flag from a boat.
Greetings, tronouk. Welcome to the forum!
Your proposed display of the US and UK flags sounds exactly right -- approximately equal size, and side by side. They could either be on staffs or on the ends of the spreaders, depending on the configuration of the boat. Since the ceremony will be in the US, the US flag should go on the starboard side.
Interestingly, I'm not sure that this display would be technically legal in the UK. I believe that the use of the Union Flag at sea is restricted to the Royal Navy.
As you undoubtedly know, it's common to fly the flag of the country of registration on the stern, on a staff, the luff of the sail, or on a backstay or topping lift. Flying a US flag in this way along with the two ceremonial flags would be perfectly in accord with accepted etiquette.
We're sorry to hear about your loss, but hope the ceremony goes well.
Best regards,
Peter Ansoff
"We live by symbols, and what shall be symbolized by any image of the sight depends upon the mind of him who sees it."
-- Oliver Wendell Holmes, Jr.