There are, and they were just as effective as their counterparts on land, only with added logistical problems. Well, as effective as their land counterparts at besieging fortifications - they weren't there to be used against other ships, except the occasional balista, and an incident during the so called Last Crusade, when siege engines brought to besiege a city weren't unloaded yet and were used as a support (can't for the life for me remember where I read it, though).

Some examples:

siege tower: http://manuscriptminiatures.com/4974/15413/

trebuchet: http://manuscriptminiatures.com/4974/15438/