Everything in Minecraft is about the odds. Most things have some sort of random factor associated with them and nothing is absolutely 100% guaranteed. It's a game. Sometimes you win the draw and sometimes you waste 5 minutes.
Spawning in water is very unlikely... but certainly possible. Since starting to play Minecraft, I have probably generated many more than 100 random world seeds. I have inititally spawned in water exactly once. On one other seed, I initially spawned on land; but accidentally exited without saving. When I re-entered the world, I spawned the second time in water.
Seeds that are mostly water are also rather rare. Perhaps 2% of the seeds I have generated have had more than 40% water. However, of the water seeds I have managed to generate, it seems that a rather high percentage of them have had only mushroom islands.
I called it "A Whole New World", and the seed is "A new fantastic point of view"
It's a lot more likely (but still rare) when you type in a specific seed. The algorithim has issues like that and I have had problems with forced seeds. If you don't put anything in the seed then the formula is much more balanced. Fewer issues and ussually have all the resources.