Thought I'd address each of your key points.
Definitely harder to work on an inboard than an outboard.
Slower speed per hp, I would say yes on diesel, but not on gas. The weight of the diesel is a factor. Williams makes one model, their 445, in both gas and diesel. The gas weighs 957 lbs and runs 45 knots. The diesel weighs 1661 lbs and runs 35 knots.
Getting fouled with sea weed and stuff. We've not had a problem, but many do. The problem develops when people run them in very shallow water. Often that first foot or 18 inches off the bottom is virtually filled with sea weed and growth.
As to interior space, a little tradeoff. You can fit a foot more jet rib into the same space as a gas. So where a 17' gas fits, you can get an 18' jet because you don't have the motor on the back. On Williams Jet's the only space taken by the engine is under the console and for steering wheel and instruments you'd give up that space on an outboard as well, although you could store under the console on an outboard.
Poor performance in rough water. That's not a characteristic of the jet but of the specific boats. Ours will perform much better in rough water than a comparably sized Whaler, better than some outboard RIB's, about the same as a Novurania outboard.
Weight on ours isn't higher. If anything it's lower. Definitely is further forward and could be a factor when loaded with maximum passengers.
In small jet tenders, the only ones I could recommend today are Williams and Avon Seasport or in very expensive perhaps Carbon Craft. In larger tenders, I like Novurania but their weight is in line with their comparable inboard-outboard conventional models so a lot of weight for a 17 or 18' rib.
As to the three jets you had, I wouldn't disagree with your comments at all. The Sea Doo was powered by a basic PWC engine, a Bombardier Rotax and underpowered compared to ours. There were also jets powered basically by Yamaha waverunner set ups. Those always performed poorly. Williams are more jet boats built with regular inboard engines and with good propulsion systems.
I never have seen a Whaler jet but can only imagine poor performance with the weight. Have no idea what engine.
Now, to the Nautica. an underpowered diesel. I hate to say this but under 20', gas just outperforms diesel by such a large margin. Now, there is some work being done with lighter weight diesels but I'm not familiar with them in actual use. We like our diesel in a 19' RIB, a 180 hp Yanmar but it's 2381 lbs too. It will hit 40 knots and a nice boat. The one model that Williams offers in outboard and diesel, we like the outboard much better.
The one negative we have that you didn't mention is fuel capacity. Most jets lack the range of comparable outboards and that's a big negative to us even though we still love ours.
The only jets we'd recommend today based on our experience are the Williams gas models up to 15' and in the 20' range, Williams or Novurania Diesel but still not the performance of the gas models, but in diesel, we'd really go with Castoldi. Weight is a big factor in a RIB. Now, Castoldi specializes in 21-33' jet ribs.
So, I don't disagree with anything you've said based on the three you've owned and I find much of what you've said to be true on other models.
Still we love ours. They fit so well on deck or platforms, they perform well, ride well. One place too where jets are dominating the market is on boats with garages. They store so easily in those situations. Most garages are on European sport boats and nearly all those have Williams Jet Tenders.
I certainly do not know about many boats mentioned but I do know about many others.
This is what I know from first hand experiences....
"Slower speed per hp, I would say yes on diesel, but not on gas."
Jets themselves are less efficient than prop boats. You can see this results when you replace a typical prop outboard with a jet lower unit.
We had a 19" Nautica with a 200 hp Yanmar jet and a 19" Nautica with a 115 Yamaha outboard. The outboard could do about 10 mph more speed and ran at a lower fuel use as well as having much more space inside.
"Getting fouled with sea weed and stuff. We've not had a problem, but many do."
We had fouled most often in brackish or fresh water right where we really wanted to use the boats. Up a bit on the Ct river or just about the entire upper Hudson you can foul in any depth of water as the 'sea weeds' are everywhere. Prop boat you just back up - jet boat you are in trouble. Same for the entire South shore of long Island and between the forks where we boat most often.
"The problem develops when people run them in very shallow water. Often that first foot or 18 inches off the bottom is virtually filled with sea weed and growth.:
We actually could run the prop boats in just about the same depth of water with the outboards being able to be 'walked' or pulled into shallower water than the jets. Running a jet in skinny water under power sounds great until you pick up pebbles and other stuff to ruin your pump and your day. It is impossible to change out an impeller as you would with a spare prop.
"As to interior space, a little tradeoff. You can fit a foot more jet rib into the same space as a gas. So where a 17' gas fits, you can get an 18' jet because you don't have the motor on the back. On Williams Jet's the only space taken by the engine is under the console and for steering wheel and instruments you'd give up that space on an outboard as well, although you could store under the console on an outboard.'
Do not know those boats but on the Nautica you sacrificed fuel as well as storage the engine takes up quite a lot of square footage
"Poor performance in rough water. That's not a characteristic of the jet but of the specific boats."
Whenever the intake grate catches air the performance will suffer - they can turn and perform well in flat water but as the chop builds the same boat in a prop version will begin to shine.
"I never have seen a Whaler jet but can only imagine poor performance with the weight. Have no idea what engine."
115 OMC based on an outboard engine.
"Now, to the Nautica. an underpowered diesel. I hate to say this but under 20', gas just outperforms diesel by such a large margin. Now, there is some work being done with lighter weight diesels but I'm not familiar with them in actual use. We like our diesel in a 19' RIB, a 180 hp Yanmar but it's 2381 lbs too. It will hit 40 knots and a nice boat. The one model that Williams offers in outboard and diesel, we like the outboard much better."
Our 19" Nautica jet had a 200 Yanmar - about 40+ mph top or so .
Our 19" Nautica with 115 Yamaha - about 55 mph top or so.
"The one negative we have that you didn't mention is fuel capacity."
Our 19" Nautica with the 115 Yamaha - 65 gallons , about 400 miles at 35 mph.
Our 24' Nautica with twin 150 Yamahas - 94 gallons , about 300 miles at 40 mph or WOT of 65 mph.
We never took our diesel Nautica on any length trips to compare.
Like I mentioned - things like ease of use, capacity, speed, maintenance, field repairs, skiing, wakeboarding and exploring were much better in our outboard RIBS in our case.
Of course you mileage may vary dependent upon usage.
