In how much water do I need to put ramekins when cooking au bain marie in the oven?
Ideally, the water level should be just as, or a bit above, the egg level. So, you fill the ramekin to maybe 1.5 cm below the rim, place it in the dish, and fill the dish until it is 1 cm below the ramekin rim. You can deviate a bit, if you must - if you had a bit too much custard and filled the ramekins more, you shouldn't get the water too high, because you don't want to get water splashed onto the custard if it should start boiling (actually, you want to avoid a roiling boil in your bain marie, but it can happen). Even if the egg is slightly higher than the water, it will still be OK. If you have the water level too low, you risk to overheat the upper portions of your custards. 

This assumes the souffle-cup-like ramekins usually sold to home bakers. If you have very flat ones, like crema catalana dishes, it gets more complicated. You have to have the water close to the rim, and bake at lower settings. It can help to put them high in the bain marie, e.g. on a cake cooling rack, so that they are cooled from below rather than the sides.