I picked up a used Edelbrock Performer 460 manifold. The ports were a much better fit to the ports in the heads. In the photo below you will see a hole in the carb mounting area, circled in yellow.

Question 1 is, what is this hole for and how can I block it off? The carb doesn't come close to covering it. Question 2 is, would the plug located just to the right of the numbers 460 be a good place to install a water pressure gauge?
Thanks.
1) That is the EGR port. The Edelbrock Performer 460 is available with that EGR port and without that EGR port. You have the EGR port Edlebrock Performer 460.
2) The port where you want to install your water pressure gauge is where the water temp sending unit normally goes, but it is not mandatory to install it here. Also, remember that this location will show pressure after the water has passed through the engine and is near your dump line, not necessarily inlet pressure.
For a jet boat, that intake is only slightly better than the OEM passneger iron intake; you might want to consider another one if it's not too late.
LO