Thanks for all the advice:
Ok, just to close the loop on this one and provide some lessons learned for others in the future see below. This was a new expereince for me, but I am sure other folks have done it many times and knew the whole time. For those new members who run into the same symptoms:
Issue:
1. Bought used F28 Trojan with twin raw water cooled Chrysler 318's that both run well.
2. Both engines discharged water when I got it a couple months ago. But I have no frame of reference to how much they should discharge.
3. Port engine blew the inboard raw water hose that goes from thermostat housing to inboard manifold while underway, but all hoses showed fatigue, so I figured ok...just replace the hoses. When it blew it started filling the engine room with water. I was alone and saw the red bilge pump light energize on the upper console. I opened the hatch and raw water was gushing out, so I knew the raw water pump was working. I inspected the pump impeller anyway and it was fine.
4. I replaced the hose, then the hose between the water circulating pump and thermostat housing blew. I suspected excessive water pressure due to blockage at this point. BUT….PO had mentioned the risers were replaced last year and they looked new to me. Also, mechanic friends of mine (not boat guys) gave me advice that steered me toward other issues. With steam in the exhaust water pump, head gasket, blown cylinder, valves, were discussed. I know better then that but got caught up in it. The steam really threw me off though.
5. I replaced water circulating pump. It took me all day on the mooring, with many wakes going by. Too much fun. With the sun just setting I crank the engine with a big smile and ……NOT FIXED!
Then I rejected all advice. Slept on it. And reflected on the simplicity of the problem. With increasing back press and weak discharge obviously there is blockage. I removed the hoses at the exhaust manifolds and cranked the engine and water flowed fine. This meant blockage in either manifolds or risers. I reconnected hoses and used a temperature gun to measure hot spots and since I could see from one end to the other of inside the exhaust manifolds clearly it is the risers. Removed risers and sure enough, completely blocked off. I was still nervous about the cause of the steam (which I still don’t understand because it makes me think water is in cylinders) I cleaned out the risers best I could (the water jacket structures are solid). This took 2 tries because I didn’t do it well enough the first time. On second try I filled the ports with water and the water didn’t drain at all. So I cleaned and filled with water over and over until they were clear. See photo. I did this because I wanted to resolve the water flow issue and see if the steam was still present to determine the extent of damage in order to order ALL of the parts i would need. With risers clean the discharge is excellent and steam is gone. Thank god I don't have to pull a head or the exhaust manifolds aren't damaged. I ordered new risers for both engines and will replace them next week.
Lesson Learned regarding above:
1. I am going to install a bilge alarm when I get a chance.
2. Going to carry new extra hoses for all possibilities.
3. Do not assume steam in discharge is catastrophic. (Although I still don’t understand why it was there).
4. Weak discharge and increasing back pressure = blockage or bad pumps. Its that simple. This should have been obvious to me and I know better then to listen to land lubbers. My own fault.
5. I will never again try to clean a riser elbow again. The was such a pain and not worth it unless you have special equipment. I only did one side. I ordered new ones.
so I know the system is getting raw water. I replaced all hoses and checked my raw water pump impeller. I checked the intake strainer and cooler. All were were fine. I replaced my circulating water pump but it did not help. The risers are new last year so i think they are ok. The discharge is only spitting water with steam coming out. The oil looks fine and the engine does not misfire at all. What I can't figure out is where is the water that is being introduced to the system going? I suspect the flow is just being blocked because I blew one old hose first and then another old hose before replacing all of them. I have no problem doing mechanical and electrical work myself. I used a thermal gun to check for hot spots but only see the port engine being 100 degrees or so hotter than the starboard engine with not difference between engine areas, manifolds, or risers on the port engine. 
