in-line 6 overheating problem

[moneyhole]

I posted some pics of my boat-only had 1 of the motor. How expensive is it to have the lower unit water pump replaced if that is my problem? I have the in-line 6 with the heat exchanger and I'm not sure of which outdrive I have as it was painted over-completely with black paint. I had the muffs on -fired the boat and after 5 mins she climbed to 160, 180 200 then I shut it down @220 degrees or so. I let it cool off and placed a tub of water covering the intake lower unit holes and it bubbled some but temp still climbed. I didn't let it get anywhere near as hot and called it a day. I really want to fix her- it's been re-wired bow to stern, engine swapped in 'o8 with a known good running engine, but I did go thru the parts today and realized they used the old engine t'stat. That was cheesy. After it cooled down, I opened the drain ports & it had no water what so ever in the heat exchanger & I'm sure the manifold as well. What are ya'lls thoughts? Greatly appreicate them.