What are you trying to do?

(boat modeltoworld [0,0,0]) vectorDiff (getposASLW boat)

produces [0,0,constant z]

Let's say that the sea is rough. Now, consider the following: _pos = boat modelToWorld [0,25,0]

_pos is in format PositionASLW, and I need a PositionASL or PositionATL with pinpoint accuracy. What do?

Try this, I wrote this quite sometime ago to substitute modelToWorld

KK_fnc_offsetFront = {
_newPos = [_pos,_dir,_offset] call KK_fnc_offsetFront;
private ["_pos","_dir","_offset"];
_pos = +_this select 0; copy
_dir = _this select 1;
_offset = _this select 2;
_pos set [0, (_pos select 0) + ((sin _dir) * _offset)];
_pos set [1, (_pos select 1) + ((cos _dir) * _offset)];
_pos;
};

_newPos = [getPosASL boat, getDir boat, 25] call KK_fnc_offsetFront;

Okay, but what if I am trying to use modelToWorld to fix the setPos/getPos imprecision? You know, that thing: #16120

Sounds familiar :) There is also getTerrainHeightASL that takes x and y and gives you z, you might want to use that too then. Again, without knowing exactly what you are doing, I would be just guessing, so I leave it up to you.

Yeah I know about that, but in all cases, there one unknown variable that ultimately remains: the height between the sea level and the top of the wave. Without it, my fix doesn't work properly over the sea, but over land it works just fine.

Still not sure what you need. If you want to know the delta wave height then:

((asltoatl getposaslw boat) vectorDiff (getposatl boat)) select 2

I need to get the delta wave height at a position without having a vehicle on that specific position.

I don't think you can get ASLW value at position at the moment, without placing an object there.

And that's the whole point of this feature request. ASLWtoASL, ASLtoASLW, getWaveHeightASL; I just need at least one of those commands.