sterndrives in salt

Quality of build between a 2006 and 2013 is the SAME with the same builder. Quality depends on the builder, not robots or any other bs. Take a look at the 40' Cabo Flybridge, can get them with shafts or props (used). Hell of a seaboat and I cannot think of a boat with better quality in that size range.

Searays from the 2000s to now have been rock solid in build quality for their class. AND, if you look at 10 year old boats, the Searays will all be in far better condition than their competitors. I've delivered Searays to places one wouldn't think of on their own bottom. A 45' sedan bridge from Fort Lauderdale to St. Croix, USVI. A 58' Sedan Bridge Fort Lauderdale to CT. etc. etc. While I respect Pascal quite a bit, he doesn't have much experience standing on the deck of a searay at the helm Captaining them. They went through a very short era, in the mid 90's with balsa coring issues, but that effected very few boats.

There is no way in hell you're sleeping 6 on a 40' in beds, unless you go to a cat. Take a look at a 43' Lagoon and some of the other power cats. Ton of room for their size and can sleep 6. Leapord power cats also have a lot of room, straight diesel inboards, etc. Most cruise at 16-20 knots.

You've owned a Sea Ray. You've liked a Sea Ray. I would strongly consider one. A 40-45' Sundancer could meet your needs well.

That 2006 - 2009 48 Sundancer is a nice choice for 6 people onboard.

Is there any advantage to stern drives with diesel engines such as speed or economy?
If kept in brackish water is it less of an issue?

It's more that the boat works better due to size with stern drives. I just ran a 35' Regal express today with volvo diesel stern drives. Engines are right inside the transom leaving more interior room than a IPS or straight shaft setup.

No, pure fresh water yes you don't have corrossion issues.......brackish no.......

Actually some fresh water lakes have a lot of Corrosion. Lakes along the Catawba River in NC flow out of the mountains and have a very high level of minerals and stern drives look almost like they do in salt water.

Old Detroit’s are also selling a good bit cheaper than 4 strokes. Lol.

And they can do double duty too, make great mooring ball anchors, LOL.

Reportedly Konrad makes the strongest stern drives, but they don't really last to long either. I looked into them for a project, didn't take to long to head in another direction.

I've heard of Konrad for a long time. I have never, ever, seen one in person. IMO 4 stroke outboards have come along so far in HP and technology that I don't see the need for a stern drive in 90% of the situations they were used in.

Depends if you want inboard diesel or outboard gas.
The Konrad is touted as a drive for diesel power.
Now there are also a few diesel outboards hitting the market, I also looked at those for the same project and deemed them unsuitable for my purpose.
In the end I decide to go Arneson's, but with not being able to freely travel and a few other reasons the entire project is on hold and may be scrapped.

The 44' Magnum only tops out at 45-48 knots. The 51' hits 60-63 knots. Also range for the 44' is even more limited than the 51.

I'm personally not a fan of surface drives but in that size range, the most common way of reaching those speeds. If I wanted a jet, which I would want, then I'd go to AB. Their 58' would have been a good choice but now the smallest shown in their line is the 80' although the new model 787 looks interesting especially as a shuttle boat. I just don't know it's size. Perhaps 78' but sure doesn't look it. I'll try to remember to ask. Sadly, like so many builders, they're pushing the larger models. 140' and 145' came in and 58' and 68' departed.

That's just the opposite of the project I'm looking at. We're removing jets that aren't repairable and replacing with Arneson's.

Jets are good for high speed planing, but inefficient at displacement speed cruising, which is the hulls new intended purpose.
Trying to go vice-versa like you want, replacing drive with jets, I think would be very difficult. Trying to design the water intake, then re-enforcing an opening that big in the hull, and getting the water flow correct would be difficult/$$. Also there has to be enough room in the engine room to have the jets intakes and pump machinery between the engines and aft bulkhead.

For the way we're looking at going it's just a matter of capping off the jet inlet, and mounting drives on transom. Since I plan on downsizing the engines I should be able to slide them back a bit and open up the ER for better service access.

Surface drives are VERY efficient. I managed a 59' Baia with Arnesons and we had it down to 1% prop slip. The Arneson drives are strong and you usually don't have any issues with them, you can get issues in the trim and steering rams leaking if you don't exercise them regularly and barnacles get on the rams. That's the good news. Now the negatives, they're very hard on motors because you are propping the boat based upon using only the bottom half of the propeller (that's all that's in the water when you're running at cruise), at hull speeds you're essentially running double the pitch. The props have to be crystal clean, if you have 6 or 7 barnacles on the props and you won't get on plane. Want to have the props checked every couple of years. Also the steering takes a lot of effort at hull speeds also because you're trying to turn big props fully in the water with an engine driven (at low rpm's) steering pump. A lot of the earlier Arneson boats were hard to get on plane. And, you have to be an octopus to get them on plane......drives all of the way up.....bring throttles up to about 1600 rpms, start lowering drives......as you gain speed start giving it a little more trim tab......more throttle.....then start pulling the drives up........then tabs up a bit......all while steering.......But they sure are a lot of fun, when you're running with that rooster tail behind you.

A friend of mine ran a then new Magnum 72' and with full fuel would only stay on plane at WOT for the first hour, then 90% the next, then 80% load from then on out........the 59' Baia I ran was the same way......a 2014 55' Uniesse was not..........The other problem with the smaller ones....is nobody has any props laying around to try different props.....Paul Sandelin (P+L Marine in Fort Laud) will be your new best friend and does excellent work at good prices, he's the Arneson guru.......Mark Wilson in Stuart,FL will be your new best friend (prop shop).