You don't think anyone on that boat is feeling numerous large accelerations?

In your scenario, if you're being propelled through the water at 100 mi/hr when the direction of propulsion is rapidly turned 90°, you bet your sweet bippy that anyone aboard would feel it. If you change your direction of motion, that's an acceleration, and if it's large at all you will feel it, quite violently is if's large enough. This has nothing to do with to do with whether the water is moving relative to the land or not, or whether the earth is rotating.

1. It's true that these guys (when making sharp turns while going fast on the stationary waters) are subdued to g-forces, as well, so if we compare two scenarios (flowing river vs stationary waters) solely concentrating on the amount of g-forces that would be applied on our "Alpha-box" (in both scenarios) in a very brief moment of time (a.k.a. analyzing (and measuring the amount of applied g-forces) what is happening in the very first second of an impact (due to the sharp turn executed in one fraction of a second), then we would be faced with the similar results which we would get from analyzing one typical crash test (as Jack Black has illustrated in his last post).

And i must admit that i have figured out (not before long) (after putting such scenario through it's paces (within both FORs)) that an analysis of a typical crash test is not going to yield usable results in a sense that we could easily distinguish the expected differences regarding what would happen (a.k.a. what would be measured) in two different frames of reference (inertial vs non-inertial).

2. So, we shouldn't restrain our analysis only to what could be measured in the first second of an impact (since if we jumped in a stationary water from a helicopter which travels 100 miles per hour we would endure the same consequences as in a scenario in which we jumped from a stationary bridge in the river which flows 100 miles per hour).

However, try to imagine our "wicked" racing team

as attempting to perform the same kind of a maneuvers (sharp turns at 90 degrees angle IN ALL DIRECTIONS (west, east, north, south) in the streaming water which flows at the same speed at which their little green-boat is propelled by the strong force of their mighty engine. Would you say that such maneuvers would be possible in such non-inertial environment? Of course you wouldn't say that.

Only you have to ask yourself : why it would be impossible for our "wicked" racing team to perform such sharp turns in a depicted non-inertial system (in which the speed of river's flow would be the same or very similar to the speed of their mighty boat)

When going upstream their relative speed (wrt river) would be let's say 75 miles per hour, and their absolute speed (wrt land) would be 0 miles per hour, when going downstream their relative speed (wrt river) would be also 75 miles per hour, and their absolute speed (wrt land) would be 150 miles per hour.

Now, you can say that they couldn't tell the difference. I already disagree, but the real problem is going to be revealed when they try to go sideways. On the calm waters there would be no difference (in whichever direction they go), but on the fast streaming river, going sideways would be a huge problem (in some aspects even much greater problem than when trying to sail upstream).

When going sideways even if they managed somehow to travel at 75 miles per hour their forward momentum would be equal to their sideways momentum, and that would be some very serious problem that every human being should have to overcome (attempting to retain their sense of equilibration).

That is exactly the problem which i pointed out in my CONCORDE argument (forward momentum would be equal to the sideways momentum).

You can object that the air is not the same thing as the water (that the resistance of air is much lesser than the resistance of water), and that this is the reason why we can't compare the dynamics of a moving objects that sail on the waters and the dynamics of a moving objects (an airplanes) that penetrate through the air.

But, then you have to face the second part of my CONCORDE argument :

*So, if the air behaved like a gas, not like a water (which presumes gradual restoration of lost INITIAL INERTIA) we should expected such outcome : Even before leaving the ground concorde would cancel out more than 50 % of it's initial inertia (momentum). What does that mean? It means that at the very moment of taking off, concorde passengers should be able to notice (very perceptibly) rotational motion of the earth beneath them assuming that the pilot of concorde right after taking off, turns concorde to the left or to the right (it doesn't make any difference), so that their direction of flight is now perpendicular to the direction of earth's rotation. Concorde passengers should be able (while concorde is restoring it's initial angular momentum (which he had before taking off)) to see VERY DISTINCTLY AND PERCEPTIBLY how the earth is turning below them from their left side to their right side (if concorde has turned to the right), or from their right to their left (if concorde has turned to the left).*Let me remind you to the first part of my CONCORDE argument :

*If the air behaved like a water, which presumes INSTANT RESTITUTION/REGAINING of partially lost INITIAL INERTIA of concorde, *passengers would be subdued (in the very moment the pilot of concorde abruptly turns an airplane to the right or to the left) to an effect of enormously strong abrupt instant sideways blow which would tend to carry concorde in a direction of earth's rotation.**You see, the second part of my CONCORDE argument deals with INSTANT restitution of partially lost initial inertia. So, INSTANT restitution of partially lost initial inertia happens in water which is a VERY DENSE medium (in comparison with the air), no matter whether we are considering fast-moving-object scenario within inertial or non-inertial FOR.

Within HC scenario (supposed closed system with "glued" (stuck to rigid earth) air) the air is treated (interpreted) more like water, than like a gas. But, we don't see such effects that would match water characteristics (within alleged non-inertial system).

No wonder, since the air is not akin to water, and the earth doesn't rotate, as well.

3. Now, i am going to show you one interesting video in which one another racing team sails 150 miles per hour upstream the river which flows at about 5 miles per hour :

At about 8 min. in the video you can see how this insanely fast boat turns 90 degrees at the full speed (for which maneuver it took them a few seconds of time, and that is why it is much more convenient example (much more similar) to our CONCORDE scenario).

Try to imagine how would such 90 degrees turn look like on a river which would flow at 50 mile per hour (1/3 of boat's speed).

At about 13 min. in the video engine fails, and the boat turns 180 degrees in a second. Try to compare this situation (that happened on the slow streaming river = 5 miles per hour), with a hypothetical situation in which the speed of river's flow would be 10 times greater, and then tell me that there would be no difference.