I am having a hard time understanding what you are trying to achieve...
Are you going to put a Mercruiser engine assembly in front of the jet? Or are you going from jet drive to a Mercruiser I/O?, you can read it both ways...
For salt water, closed cooling is most definitely the way to go, I have done literally dozens of these, including building the heat exchangers from scratch.
I would highly recommend if you are going Mercruiser power package (drive and all) I would try to get the pricing difference between closed cooling and open, a lot of times it is way less expensive to do it as an option as it is to do later.
If memory serves me (rarely does) it is like a $1500 kit but can be added to the package for less than $900 (pricing is old, but you get the idea.
I can build a heat exchanger itself for just over $850 and for an additional $100 can build it with an integrated oil cooler... All 100% Copper and in custom configurations.
Let me know exactly what you are trying to achieve and I can get you turned in the right direction...
BTW, it takes 3-4 weeks to complete a heat exchanger depending on how crazy you want it...I'll get some pictures of what I am talking about, maybe as soon as this weekend...
GT
