I have an A15 with the wet cylinder head type, but have a dry type inlet manifold.
Can I just simply fill in the water holes on the A15 head to match the dry type inlet manifold I have, or do I have to source a wet inlet manifold to match the wet head?

Yes you can, I tapped threads and screwed in set screws with some thread sealer on them.

Thanks, that saves a lot of grief.

NISMO used to sell a stud and plug kit for side draft carbs; the plugs were nothing more than two pairs of set screws. As mentioned you simply tapped some threads in the head, put some Teflon tape or epoxy on the screws and screwed them flush with the head.

Couldn't you just install a dry gasket that blocks it off?

My manifold didn't cover off the water ports. Maybe the dry type manifold is the same?

Some of the aftermarket manifolds will cover the ports -- like my dual-sidedraft manifold. I used a wet gasket with a dab of RTV over each water port. But other manifolds don't cover the area, that's when the blind screw plugs are need