When you go about filling a gap like that, might I suggest:
Try to fill as much as you can with sheet styrene. There should be some laying around that can be slid edgewise into the gap. It might only be an inch or two at the widest area there at the very bow. Look around for stuff like the top of a clear plastic clamshell food container and cut a piece about 1/2" wide and as long as it can be and still fit it into the gap. Slide it in there with some sticking above the deck. Flood it with solvent glue on both faces, making sure it doesn't run down the hull.
After it is hard, take an Xacto or a razor blade and tgrim off the extra as flush with the deck as you can.
Now mask with tape along both sides of the gap, leaving a very small extra area along each side of it.
Run a somewhat abrasive sanding stick up and down a few times to make it flush without destroying the tape.
Squirt out a little putty onto something you can scrape it up from, like a length of blue masking tape stuck to the bench top.
Take something like the scrap that you trimmed off and scoop up a little putty. Trowel it onto your gap between the edges of the pieces of tape, using the thickness of the tape to allow you to "screed" it flat. I save my dull #11 blades for this. Holding them by the tip in the handle, the square end of the tang makes a great little one-use tool.When it is dry, another light pass with a medium grit stick, and remove the tape.
Do a little fine touch up with a fine grit stick. The edges of the tape will have created little ridges that you'll need to get rid of.
Prime.
When dry take a look at it and try to touch up any rough spots.
I learned a while ago, the hard way. Arbitrarily loading a seam or gap with uncontrolled application of putty takes out all kinds of surrounding detail when you go to remove it. And starting out with a hump of putty just creates unnecesary extra work to remove.
Go to it Chief,
Bill