Hello All, First off let me start off by saying that I am a total Newbie and just bought my first jet boat. It's a 73 Rogers with an Olds 455 and Berkeley Jet. I have a very limited history on the build and the hours of the engine and the pump. I've had it in the water a couple of times and it seems to run really well. The main issue I have is pressure in the cooling system. It has Bassett OT water injected headers too.

It had a rat nest of plumbing and valves all over that I am trying to clean up. Currently, off the pump, I have CP Performance Relief , to 3/4 gate valve, to a 5/8 x male hose, 3/4 x female hose to 5/8 supply. tee's to 1/2" hose into the block, out of the thermo housing one side is 5/8 hose to an inline valve that then goes out the transom. The other side is 1/2" hose to an inline valve that tee's at a Bassett header valve. No matter what I do my pressure reads just under 15 when cruising lower RPM, more I throttle up pressure increases fast. I'm afraid to give it anymore in fear of blowing gaskets or causing any damage. it's jumping to 25-30 which from what I keep reading is not good. Any and all help is greatly appreciated.

As stated above, the primary discharge should be an open line. Also have someone watch the pressure relief line while at high RPM to see if its opening. Your relief may be stuck shut or plugged with sand.
If you want to regulate engine temp, look into a thermostat kit. I think its a better solution than throttling the incoming water flow.  I put one on my 454 and it runs a constant 155-158 and no more cold stumble out of the hole.
-- Looking at the picture, that pressure relief valve appears to be adjustable. It's not just screwed in too far is it??