I did some testing and found out some interesting things.
Firstly, there doesn't seem to be a way to really escape the map vertically-- there is a safe area, but it doesn't cover the whole map and it's very short, it's small in vertical space.
Secondly, the soft kill zones I talked about seem to mostly be one big soft kill covering one end of the shore.
Thirdly, there are hard kill zones in the water in some areas-- I was doubtful of this for a while, and several of what I thought were hard kill zones seem to be holes in the map, but there are hard kill zones.
Lastly, the kill from going too far out seems to be from walking off the edge of the map rather than a hard kill zone-- there's a steep drop around most of the map which can be walked on safely and which leads to a well-defined edge you can see if you look down.

I marked the safe and unsafe underwater areas once I'd figured them out and made a clip to show the safe area. Everything from one kill ball to the other (around the end of the map with the crashed dropship) is unsafe underwater, and everything from one tin cup to the other (the rest of the area just off the beach) is safe underwater, with some seemingly rare exceptions. The modifications seen here are in my file share.