What you calculated are not the chances of the opponent not beating you, you calculated the chance of getting the first hit. But it is the last ship being hit that will make you lose.

Imagine player A using a regular strategy, and B using this stacking strategy. Now imagine that B gets insanely lucky and sinks all of A's ships except their largest one, without missing any at all (while A keeps missing all their shots). So now, even after all that insane luck from player B, who has a better chance of winning? At this point, they both have only 5 spaces which can be hit, and any hit will quickly end the game, so they both have 50% chance to win right?

This means that clearly if B doesn't get insanely lucky in the first turns of the game, then A will have much better chances to win. A's chance to win reduces to it's absolute minimum at 50% when it has only one single 5-length ship left.

If you had a ship on every possible space, they would have a 100% chance of landing a hit, so by your logic this would be worse. But it would take 100 turns to sink you, which is the best possible outcome.

Having only one 5 space ship is objectively worse than having one five space ship and all the others. Unless of course you also created a new rule that they have to hit the same spot multiple times, once for each ship.

At that point you’re just playing Calvin Ball, though, and you might as well put the ships under your chair and claim “You never said floor!”

When the smallest ship (can't remember what its called) is destroyed, you would then tell your opponent that they sunk it. It would be unlikely that they would continue to strike the next space in that row/column.

Does 1 hit cause damage to all the ships in the stack that occupy the hit space? Or does only the top-most ship take the damage such that the top ships shield the bottom ones?

I'll add to this, it's not just about finding the initial probability since the probability for event B given A, or event C given A and B, these would increase the liklihood of being found. I couldn't tell you exactly what the probability is but say for example event B given A, it would be 1/n for n being the number of legal spaces around A. So if the ship was in the middle of the board it would have the smallest probability of being found but if you're along a wall, there's only 3 possible legal spaces so the probability increases to 33.3℅, and if A was sandwitched between two walls, the probability of B is 50℅.

so if there's a moral to this story, assert dominance and put all your pieces in the center.