I don't know your boat size but don't get too large of an engine. Diesels are happier being driven hard, having unused horsepower, especially with a turbo is not healthy for the engine.

The turbo kicks in at higher RPM. If you don't get that high the turbo will coke up and won't work when you want it to. I knew a sailor with one on a Sundeer 56 and he had nothing good to say about it.

I used to worry about water in the engine after long downwind passages but it never happened. My exhaust has about a 30 inch drop from under the deck to the outlet. Rather than fitting a valve which won't like hot salt water consider a flapper on the outlet. That combined with the vertical drop and the water lift muffler should keep your engine dry. I hope you don't have a mid cockpit boat with a side exhaust - those seem to be the worst for getting water in the engine.