If you make too big waterblocks there will not be collisions for the whole water, I do not recall exactly where this limit goes, but I think it stops at 7000 in size, anything bigger you will be able to just walk through.
Since you mention it's going to be a big map, guesses are your waterblock is too big to have water collision around the whole area.
Doesn't even have to be that, maybe you didn't centrate the actual middle of the water block to the middle of the actual "lake". When placing water and the water block is actually bigger than it has to be, try to always place the small block in the middle of any lake/water and then edit the size and height outwards to your likings.
If this is not your problems I'm not sure what would be. But if the water doesn't reach, you can always make several blocks if you line the up nicely, just to copy the same height and diddle with the arrows.