It's been well over 5 years since I touched Flash (possibly more...), let alone actionscript.
But should be easy to do something like that, assign name to button, use actionscript OnClick event to take to another frame (where the "hello there!" is in) and OnRelease event to take you back to the original frame.
Since you want the fish to keep moving while clicking, you should use the object "internal" frames (I forgot however it's actually called >>), I think you have to double click the object or something like that.
Hmm, I think you can also assign different states to a button through the UI, which would make this even easier.
edit:
http://kb2.adobe.com/cps/126/tn_12632.html See if that helps.
If I had flash installed I would probably figure this out in 10 seconds, but since I haven't used it for so long it's not. Hopefully I was able to help though.