A titanium plate heat exchanger may be more cost effective, since you want to use titanium.

We need some guys who work on fishing boats to comment here, but I'll add my two cents...

The use of titanium is recommended for fish. Apparently, if you use stainless steel or copper the fish are sensitive to those metals. Titanium is basically inert as far as the fish are concerned.

I seem to remember this from a project for lobsters. It should be similar for other fish, unless, if you will pardon the pun, I'm all wet.