[Babul]

Code:

new Float:XPos,Float:YPos,Float:ZPos;
GetVehiclePos(GetPlayerVehicleID(playerid),XPos,YPos,ZPos);
if(ZPos>-1 && Zpos<1)
{

then the vehicle is at water level...
if(ZPos>-1 && Zpos<1) works when the sea is calm, so idk if it works all time (erm.. at each weather)...