Edit in JSFiddle

function initialize() {
    var myLatlng = new google.maps.LatLng(51.24343506423872,22.56488800048828);
    var tylkoWoda = [
      {
        stylers: [
          { visibility: "off" } // * Usuwamy wszystkie elementy z mapy.
        ]
      }, {
        featureType: "water",
        stylers: [
          { visibility: "on" } // * Pokazujemy tylko wodę.
        ]
      }
    ];

    var mapOptions = {
      zoom: 4,
      center: myLatlng,
      mapTypeId: google.maps.MapTypeId.ROADMAP
    }
    
    var map = new google.maps.Map(document.getElementById('map-canvas'), mapOptions);
    map.setOptions({styles: tylkoWoda});
}
    
google.maps.event.addDomListener(window, 'load', initialize);

<script src='https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false'> </script>
<div id="map-canvas"></div>

#map-canvas{
    height: 400px;
    width: 400px;
}