Yes, the water check occurs on landing or 'spell finishing'. It does not occur on initial cast.

The comments about the person moving into the water were helpful, but not technically what was happening. The real issue was the LOS.

So for example currently on Red99, let's say two characters are in RV by the pond, both out of the water, and one starts casting on the other. If the target jumps in the water and swims into the middle, where the caster still has LOS to him, the target is 'protected'.

If the target jumps in the water and swims up against the edge, where the caster doesn't have LOS to him, then Null's LOS code kicks in and the spell lands. This is fixed so that in the next patch, the target is still protected.

Aw yes I noticed that happened and was thoroughly confused because I thought a) I was in water how did it land and b) he didnt have los how did it land

Fixed.