what about vertical turbines in the water & bolted to the sea floor?

All engineering at sea costs more. Seabed anchoring is a thing, and a higher degree of wind offshore is a thing, but if you do the linear optimisation of the different cost benefit lines I suspect scaling up traditional fan style 3 blades just wins.

It's a "perfect is the enemy of good enough" thing. Better designs along one axis with a multi axis problem won't be best overall.

Doesn't change a thing on how stresses damage the turbine.

Vertical still only makes sense in limited space.

Interestingly, vertical designs have been proposed for Mars, because space for shipping them is at a serious premium, and the much lower atmospheric density reduces the stress considerably. (Hollywood depictions of destructive Martian sandstorms are quite exaggerated for effect.)

Mars is a good example of differing constraints leading to choice of different technologies, indeed.

what are the stressors? mechanical failure, too much kinetic energy in the waves?

The pole in the center of the VAT generates a turbulent wake that the blades pass through once every rotation. This wake shakes the blade and causes extra stress. The blades themselves make a wake also, though not as much as the pole.

The horizontal turbines put the pole behind the blades. The pole does make a bow wave, but it is not as bad as the VAT wake. The pole/blade interaction is not as severe.

thank you