after 9 pages, i thought I would start afresh. I am going to change my mind and argue that those two have the option of being taken as non-dedicated transports.
Through out all the codices I have read so far (and reread for about the fourth time), even when the transport option is included in the troops section it says in the description that it is taken as an upgrade. Neither the fish or CSM rhino does. The BBB differentiates between dedicate and non-dedicated specefically. Many entries in codices are specefically written to say that they are "dedicated"or "unit upgrade". These two aren't. We cannot really use intent, because we can have two opposite opinions of intent that carry equal weight.
What is left? Blasted RAW.
No where does it say in the CSM rhino entry (in the elites section) or the fish entry that this limitation applies. I went into my local FLGs and posed the question and got laughed at for being stupid. We then got the codices out - and this really surprised me, they all changed their minds. Once intent and background where rejected, the RAW was left.
I think the may problem is, like everyone in the FLGS, is most people think it shouldn't be. However that is not what RAW stated.