I put a deck on the front of my v-hull and were it meets the sides there is about a half inch gap.  I was wondering what I could use to seal up around it to help keep water from getting under it.  I am going to be putting roll on bedliner over it but I don't like having the gap there any suggestions would be great and thanks in advance.

1/2" gap is a little to big for any typical caulk. Try backer rod then caulk over it, or an expanding foam like great stuff. If you use great stuff, let it cure then slice it even with the deck with a sharp razor blade because it will expand past where you want it. Hope that helps.

Sounds like me when I used to do upholstery, I think I'm the only person to cut something too small,.....twice,lol.

Now, I'm all for doing it right but if there is a reason it had to be cut short, try wedging something in there.

Think round.  Something with the same diameter as the gap that you could wedge in there.  Hose, rope, etc, something like that.

The spray foam stuff works great, Lowe's & Home Depot sale round foam of various diameters that comes in rolls.

Good advise so far.  The backer rod mentioned by Fishin Phill is a foam rod made just for what LBH is suggesting with the rope, hose etc. It is also what Catt is talking about that comes in roles. Did I leave anyone out? You will have to put caulk over top of the backer rod after it is in place.  The backer rod is just to support the caulk while it's wet.

If you go with the great stuff expanding foam after you cut it flush with a sharp razor it will have a rough texture from the air bubbles inside it.  You can use regular sparkle to smooth it. Covering it with the bed liner paint will give it the water proof that the spackle needs.

Foam caulk backing and 3M 5200