The most notable ship preserved in Cardiff Bay, or at least the easiest to access until 2015, was this light ship, called variously Lightship 2000, Lightship 14, Goleulong 2000, and Helwig. Built in 1953, it was literally a "light ship," which served as a floating lighthouse to prevent other ships running aground. In spite of the development of radar before its construction, it remained in service until 1991, before being bought by an entity connected to the Cardiff Bay Development Corporation (I haven't been able to find a source specifically naming the buyer). For a time, it served as a chapel upon preservation in 1993, until it was closed to the public in 2013 and moved to Gloucester in 2015, with an eye to opening it as a museum following another restoration (which it has also served as over the years), but in Newnham-on-Severn, rather than Gloucester or Cardiff. At the moment, it is supposedly is being restored by Fred Larkham, although I have been unable to verify the owner. Their is an official website for the ship at:

www.lightship2000.co.uk

...but the site appears to be down at the moment (June 12th, 2016).