I know I'm going against the advice of many very knowledgeable people here, but I would not recommend a boat with a tunnel. I ran jet boats in many rivers of Alaska guiding for 30 years, and in my opinion flat bottom is the way to go with a jet boat. You need around 3 inches of water to run a jet boat, regardless if it has a tunnel or not. When the tunnels first came out many lodges in Alaska bought in to the new design, and now 25 years later, none of those same lodges use tunnels. I was so excited to get my first tunnel, being senior guide at the time, I had my pick of boats. I picked the brand new tunnel River Song lodge had just bought. Half way through the season, I switched to an old beat up flat bottom, and since then, I have only used tunnels when I had no other choice. The motor will be the first thing to hit bottom in a flat bottom boat, and if you don't have the motor locked down, most of the time, all that will happen is the motor will kick up and no damage will be done. You may pick up some rocks in the grate, and have to stop and remove them, but many times, by the time the motor goes back down, you are in deep enough water with enough speed to keep on going. In a tunnel boat the first thing to hit is the bottom of the boat, and even thick welded boats can get a whole in the transom been there done that. Tunnel boats take longer and require deeper water to get on step. They are less maneuverable in very tight quarters because the tunnel wont let the back of the boat slide as well. The water in the tunnel acts like a rudder making the boat more stable, less prone to slide, which can be a good thing, but you don't want to meet someone head on, and need to do a quick 180 in a tunnel boat, you better hang on tight, because it is very difficult to make a quick slide and change direction on a dime. You would think carving a tight turn, would be better than the back sliding, but if you really want to turn sharp corners, being able to slide the back end around a turn, will get you around tight spots that are impossible to make if you can't get the back to slide well.
Another thing to consider is, don't rule out riveted boats. Welded boats are strong, and last forever, but are also heavy. A heavy jet boat requires more power to get on step, requiring a bigger engine, which means you have to go faster to remain on plane. In tight rivers, you want to be able to remain on plane at as low of a speed as possible. Big inboard jets look cool on videos, flying down skinny rivers, at break neck speeds, but if you want to operate in the smallest water, safely being able to remain on plane at the slowest speed, is what will get you to your destination. Another advantage to less weight is the amount of time it takes to get on step. Many times you only have a few yards of deep water in a hole before the next shallow riffle. If the boat is not on plane by the time you arrive at the riffle, you will come to a quick stop, with possible damage. A jet will go in 3 inches of water on step, but takes much more than that if not on plane, and even more while trying to get on step. Riveted boats are light because they use aluminum that is thin, and a type that can't be welded. That is why you can't repair a leak in a riveted boat by welding. I didn't know this until I had to help repair airplane floats. We always had to make patches and rivet the floats, which was very difficult to do. I would have to crawl inside the float with a bucking bar, while the mechanic would use the air gun to buck the rivets from the outside. The mechanic was an expert welder with aluminum, and one day I asked him why he didn't just weld the leaks in the floats. He told me that even though he was able to weld very thin aluminum, like he did with the fuel tanks, which were extremely thin, the aluminum in the floats was not the type that could be welded. They don't use the type that can be welded, because it has to be thicker in order to get the required strength, which means heavier, which makes welded floats and impossibility for float planes. He then laughed when I mentioned our lack of success welding aluminum boats. Basically if you want a lighter boat get a riveted boat.
Welded aluminum boats with tunnels, are great boats, and I am not saying they aren't. I am simply pointing out, that spending a lot more money on an expensive welded boat with a tunnel, does not necessarily mean you will be able to get further up a river to unfished waters than with a less expensive flat bottom riveted boat. If you don't want to use a jet boat to the extreme limit, than get a heavy welded boat. I still wouldn't get one with a tunnel. The tunnel only makes the boat slower, take longer to get on step, require more fuel, and doesn't give you the ability to run in shallower water than a flat bottom.
If you own a tunnel, and disagree with my opinion, I will ask if a tunnel boat is the only type of jet boat you have driven? If you have thousands of hours on both flat bottoms and tunnels, then please let me drive you tunnel some day, I would like to be proven wrong and would love to find a boat that will go in less than 3 inches of water.
I believe the best riveted boats are made by Lund, but they do not have a model, that makes a good jet boat. I recommend a Lowe 1852 flat bottom, with a 40 horse tiller engine. A 40 horse engine will only put out about 25 horse power with a jet on it. Some companies now rate the horse power at the jet, so the horse power rating marked on the cowling can be as highly optimistic as 30 horse, but is still the same 40 horse engine if it had a prop.
I realize my opinions formed by specialized situations on certain rivers in Alaska, may not apply to the water you want to fish. Others who have far different opinions, that fish in the waters you are wanting to go to, may be the people you should listen too.
I just don't want you to think, you have to buy in to all the bells and whistles in order to access great fishing in shallow rivers. Sometimes simpler and less expensive can be better.