I don't think any extensive analysis has been made on the subject, but given their reptilian ancestry I'd say they would be waay less intelligent than dolphins, which are by themselves quite extraordinary amongst non-primate mammals. I would guess Ichthyosaurs were a bit smarter than crocodiles, at most.
Gosh I can't even begin to tell you how much I love all of your paleo art. You truly bring them to life, better than I have seen anyone else do. Amazing work - as always! Also cool for choosing the colour scheme of a Pac. white-sided dolphin for these guys