石が窓ガラスに当たった
The stone hit [struck] the windowpane.
In the primary definition, the subject of the verb is the hitter, not the hittee...
but as I say, it has a dozen or more uses many of which make the hitter the subject.
Your example looks like this primary definition and makes the water the subject of 当たる, and トート the object... that may be what's actually happening, I'm not quite sure, but as I say, I expected the -subject- of -both- verbs to be the same and not to change.
(I also pretty much decided it wasn't worth fussing over which is why I deleted the wondering the first time. At some point I should certainly look at some examples to specifically see how often the subject shifts around, but it wasn't worth it to me to do the other evening. Maybe tonight.)
It's also possible that トート is the topic throughout and fills in the role of subject on the left side of the conditional and object on the right side. (トートは)(水を)よけなければ、(水が)当たる ->(トートが)(みずを)よけなければ、(水が)(トートに)当たる。
I'm kind of leaning towards that at the moment more or less because even with twelve definitions I can't really find one that works to keep トート the subject on both sides.
(This is of course, purely a question of 'how is the grammar put together', it doesn't at all change the understanding of the sentence.)
(Edit after a little haphazard research
Poking around the web a bit hasn't helped resolve this, heh. I do find some sentence fragments in alc like 'be hit by a stray bullet' that support the notion that 当たる can be used the other way around,
hit by a stray bullet
《be ~》流れ弾に当たる
And I also find that there's no rule or tendency to avoid switching subjects as you cross a conditional. A bunch of the tae kim sentences change subject, as do a couple entries in the progressive JE dictionary,
水がなければ生きられない
We can't live without water.
Obviously the subject changes from water to 人間 or 生物, possibly clarified by a topic if it was actually in context.
In other words, it looks like it could be valid to interpret it either way, and that in any case any notion I had that the subject should not change across a conditional was mistaken.
Edited: 2012-03-15, 6:55 pm