I concur on Splash Zone epoxy
putty by Z-Spar. It comes in two shallow cans, one ingredient is yellowish, the other blackish. Open both the can's lids, dip your hands in water (seawater is fine), then dip your hand into one can, scooping out a golfball sized portion, dip the other hand into the other can and extract about the same size glob. It's not exact, and doesn't have to be, it's VERY user-friendly. Mush the two parts
together, continuing to wet your hands to keep the goop from sticking badly to your fingers, until you get a uniform color mass, kind of an olive drab, about the consistency of Play-Dough. Smoosh the stuff into cracks, or roll it into spaghetti, or simply pile it on top of things, on top or underwater, pressing it onto the substrate until it sticks well. Continue caressing, fondling and cavorting with the mix until it molds into the desired shape. It takes a couple hours to harden, so come back to it from time to time to remold or remind it of its future form. After it cures hard it can be filed, sanded, epoxied, or painted (though I wouldn't hold out any hope for underwater painting).
I use the stuff for creating fairings around transducers, I cover my rudder hardware
and shaft strut with it to make streamlined shapes that are easy to clean underwater. I use it for cosmetic and structural fills, and carry it as one of my stalwarts in the damage repair kit. I especially like it for setting stantion bases and pulpits because the bases almost never match the deck
camber. Simply place a ball of the mix under the base, temporarily bolt the base down, and with water, smooth the excess into a sculpted form that looks like it was planned that way. Later, sand and paint the base, then seal it as you would the deck
If you have leftovers, play with it. You can make some fun stuff on your boat such as drip lips on topside throughhulls, great fillets, or even a sculpted figurehead (one of my future fantasies).
Just remember, when you're done, clean those fingernails diligently or you will not get invited to too many parties. It's something about reaching for the snacks with hands that look like they've been in some nasty stuff.
The note on the polyurethane
sealants is possibly even better for your application, since it flexes. Spash Zone is very hard when cured and wouldn't be a successful in a planked, aging hull