I have a marine 454 with no risers. The question is. Do the stock risers stop water from going into exhaust headers? Is there a one way valve in them?

No valve. The angle and exhaust gases force the water out.

I'm still piecing this engine together. Do I need the water hose to the riser and to the bottom of the header? Can I block the riser hose hole? Will water flow from the bottom header hose up through the riser ? Thanks Mike.

Yes you can block off the hose to the riser. Yes it will flow up through the manifold.

what everybody else said,mercruiser had several different thermostat housings..Some had 2 hoses and some had 4 hoses,you will need to ger the rubber caps with hose clamps or remove the nipples in the risers if you can get them out and install npt plugs..Then route the hose from the thermostat housing the the bottom nipples on the exhaust manifolds...It the nipples are plastic on the manifolds get rid of them and go with the brass ones..

check the inside your exhaust tips for flapper's,this keeps the water from waves rushing in when the engine is off..If their is none their you will need to either get the external flappers that are held on by a hose clamp if you have straight tips or if their angled you will need to have some installed internally..