I am tempted to say that like s-x, the bigger the better (and also the harder the better) but of course that would not be the done thing
I am not a fisherman, so cannot comment on that activity (or is it lack of activity
The comment about engine size is relevant, but if you are going to dive off of it, you really do need a bit of space. Will it be just yourself who will be practicing bubble blowing, or do you do it in concert? If just yourself, I would say 5 metre minimum, and if two 5.5 metre minimum.These would give you enough space to kit up properly and in reasonable comfort, depending on the internal layout of seats, console etc. No doubt you will get other views, but this is what I have found when diving off of RIBs.
Your description of the seas out there has got me ever so slightly jealous.