I want to convert my raisers to from the stock wet exhaust manifolds to a dry exhaust. Has anyone tried drilling and tapping the factory cast manifold to add a water transfer port?

Ayuh,..... You mean dry Joint,..??

I'd just swap the exhaust, 'n sell yer old ones,.....

There should be a pipe plug in the manifold near where the riser bolts on, see if you can get that out and thread a hose barb in. Whatever risers you put on will need to block the water jacket in the manifold.
You won't really see any performance gain from this but it will be louder, but might sound more like dump truck with no exhaust loud not race boat loud, because of the stock manifold design.

omg lol. Dump truck loud... good one👍

What I'm talking about is replacing what is in the red box with something similar to the image in the left. water still has to transfer, so drilling and tapping where the blue dot is.

Yeah that'll work fine. Is there not a pipe plug anywhere in the manifolds you have? Just saying if there is you don't need to drill or tap anything, just thread a hose barb fitting in.

No there is no plug in the manifolds I have or I would have just opted to used them vs drilling and tapping. The overall attempt is to get a O2 sensor in the exhaust stream in a dry section

Imco is only one company that makes risers like you posted to bolt up to stock Merc cast manifolds and they use a wet joint.
What exactly are you trying to accomplish??

Yeah that might be a problem...unless you are custom making a riser or something. If you're trying to save money I would just look for a used set of aftermarket manifolds, EMI, Stainless marine, etc. Save you a lot of work, and be lighter than factory cast iron.

I have adapted Gil riser's to small block Mercruiser manifolds, with home made adapter plates
so it can be done