[Mean]

First include this.

Then add anywhere in your script:

pawn Код:

public OnPlayerStartSwimming( playerid ) {
    SetPlayerHealth( playerid, 0.0 ); 
    return 1;
}

If that doesn't work, try putting this in OnPlayerUpdate (or a timer):

pawn Код:

public OnPlayerUpdate( playerid ) {
    if( IsPlayerSwimming( playerid ) ) {
        SetPlayerHealth( playerid, 0.0 );
    }
    return 1; 
}

Simple, ain't it?

EDIT: By the way, it doesn't work while you're in a car.