Your headers will get the same amount of water regardless as all your cooling water exits thru the headers, however your motor may not get as much as it needs under extreme conditions.

When the thermostat is closed, the water exits through the check ball outlets, then flows to the risers, skipping the exhaust manifolds. When the thermostat opens, the check balls provide some back pressure to push water throught the block and out the non-check ball outlets. These are typically plumbed to the exhaust manifold. You NEED the check balls to get good flow through the block.
In later years, Merc redesigned the thermostat housing to eliminate the need for the balls.
Gary

Gary,

I believe he has after market headers with the two exhaust hoses on each side y-ed together. Thats why I said that all the water will go to the exhausts either way.

Yep. you're absolutely right regarding water to the headers. But he still needs the checkballs to prevent too much water from bypassing the block when the thermostat's open.