I have email into both Peter Orton and Aled Williams and have been awaiting response. Peter did respond with a promise to check on dates etc..., but has a lot on his plate at the moment ;-)
I might suggest some adjustments to the family tree. The Aquanaut and its kin were developed from the Avocet so might more accurately be placed as descending from the Pintail rather than the Nordkapp. In order it would be: Pintail, Avocet, Argonaut, Aquanaut. The Argonaut (a.k.a. Aquanaut HV) came before the Aquanaut which was first offered as a lower volume version of the Argonaut. In addition, Aled Williams has stated that the starting point for the NDK Romany was the Pintail, so the Romany and subsequently the Explorer could be placed on the tree descending from the Pintail.
Also, my understanding is that the Nordkapp H2O is a modification of the Jubilee, not the original ('Classic') Nordkapp. So the line of descent would be; Nordkapp, out of which came the Nordkapp Jubilee, out of which came the Nordkapp H2O. Though the H2O has a number of changes which differentiate it from the Jubilee, my understanding has been that the H2O designation was only used on this (American) side of the pond.
http://www.ukriversguidebook.co.uk/forum/viewtopic.php?t=9711