Floating on water

I am trying to make an avatar to float in a water pool without any need for avatar to have an attachment. I know that pushing an avatar is not correct, but the script should apply a small push only until the avatar is in the water, to make it float. Pushing force is calculated according to Archimede's principle. I wrote a script based on DetectVolume and collision_start and collision_end but it looks like I cannot obtain the right effect, and furthermore collision_end is not always detected. Here is code (any help appreciated):

list objects = [] ;
float GRAVITY = 9.8 ;

float force(key k) 
{
    float f ;
    key me = llGetKey() ;
    list obj_box = llGetBoundingBox(k) ;
    list me_box = llGetBoundingBox(me) ;
    vector obj_min = llList2Vector(obj_box, 0) ;
    vector obj_max = llList2Vector(obj_box, 1) ;
    vector me_min = llList2Vector(me_box, 0) ;
    vector me_max = llList2Vector(me_box, 1) ;    
    float ratio = (me_max.z - obj_min.z)/(obj_max.z - obj_min.z) ;
    if (ratio > 1) ratio = 1 ;
    
    if (ratio > 0)
    {
        f = ratio * GRAVITY ;
    }
    else
    {
        f = 0.0 ;        
    }

    return f ;
}    

addKeyToList(key k)
{
    integer pos = llListFindList(objects,[k]) ;
    if (pos == -1)
    {                      
        objects += k ;
    } 
}

removeKeyFromList(key k)
{
    integer pos = llListFindList(objects,[k]) ;
    if (pos != -1)
    {                
        llDeleteSubList(objects, pos, 1);
    }      
}    

default
{
    state_entry()
    {
        llVolumeDetect(TRUE) ;
        llSetTimerEvent(0.1) ;
    }
    collision_start(integer num_detected) {
        integer i ;
        for (i = 0; i < num_detected; i++)
        {
            key k = llDetectedKey(i) ;
            if (k != NULL_KEY)
            {
                addKeyToList(k) ;
            }
        }    
    }
    collision_end(integer num_detected) {
        integer i ;
        for (i = 0; i < num_detected; i++)
        {
            key k = llDetectedKey(i) ;
            if (k != NULL_KEY)
            {
                removeKeyFromList(k) ;
            }
        }    
    }    
    timer()
    {
        integer length = llGetListLength(objects);
        if (length > 0)
        {
            llSay(0, (string)length + " objects in list") ;
            integer i ;
            for (i = 0; i < length; i++)
            {
                key k = llList2Key(objects, i) ;
                float mass = llGetObjectMass(k) ;
                float f = force(k) ; llSay(0, "Force is "+(string)f);
                if (f > 0)
                {
                    llPushObject(k, <0,0,1>*mass*f, <0,0,0>, FALSE) ;
                }
                else
                {
                    removeKeyFromList(k) ;
                }
            }
        }
    }
}