Get all seamarks of Rostock from Open Sea Map (OSM) using the Overpass-API.

gistfile1.txt

URL:   http://overpass-api.de/query_form.html
Query: (node(54.085, 11.867, 54.2503, 12.320);<;);out;

This returns a file roughly 45 MB in size. 

Extract the seamarks stuff by using PHP:
<?php
echo '<?xml version="1.0" encoding="UTF-8"?>
<osm version="0.6" generator="Overpass API">
<note>The data included in this document is from www.openstreetmap.org. The data is made available under ODbL.</note>
<meta osm_base="2013-08-15T07:48:02Z"/>
';
preg_match_all ('#\s+<node[^<]+/>|\s+<node.+?</node>#si', file_get_contents ('full.xml'), $aHits);
foreach ($aHits[0] as $sHit) if (preg_match ('#seamark#si', $sHit, $aHit)) echo $sHit;
echo '</osm>';
?>