Money, large denominations.
Oh, you don't mean for me.
There's free to expensive ideas below.
First boat; Stainless steel folding
rigging/sailing knife with the boats name engraved on it.
old salt; a old brass
compass or ships bell (antique, same name if you can find it, blank if not).
well equipped; gift certificate for x hours of you waxing the boat with the owner

- no money out of pocket and always needed.
Bare boat; a set of signal flags, and a flag halyard (installed), plus a book of flag etiquette.