For a .130 (3/16) flex shaft
& .15 pro traxxas marine engine and a sidewinder mono Hull, what prop would suit my needs?

A .130" is a hair over 1/8", 3/16" is .375". The max size (using Octura #'s) for a 1/8" is their X-435. For the 3/16", they start at X-437.
I could recommend props but you'll find out that you have to get a prop that the boat is happy with. Whether or not you have a surface drive helps to let us know what size to start at.

Its surface yes

The Traxxas motors seem to be able to run a larger prop than most. You might want to start out in the neighborhood of an X-438 or X-440/3.

OOPS ! Re post #2----3/16=.375 ???? I coulda sworn............ Best to ya !!!
Gordy