The main part of Cardiff Bay has no speed limit unless there is a rowing event taking place (which there wont be any now until next year).
Both rivers have a 5 knot speed limit for their entirety (i.e. from their mouth all the way up until you go aground) and also the Inner Harbour (Mermaid Quay) has a 5 knot speed limit.
These limits are enforced by Harbour Patrol boats & Barrage Control
It can get a little choppy, but not very often. It all depends on the wind direction & strength
and also how many gin palaces are buzzing around :P
Fishing is not currently allowed within the confines of cardiff Bay and the rivers, however that is changing soon with rod licences maybe being issued starting next year.
The Cardiff Harbour Authority website is the best place to get information on what you can do in Cardiff Bay - http://www.cardiffharbour.com