Regarding the marine ISB and SSB before the 1970s, it seems not easy to estimate to what extent it was used.
I suspect that the Caronia installation may have started a trend for ocean-going passenger vessels that were often out of the range of VHF shore stations.
The Canberra was another example. Information here:
http://www.seadogs-reunited.com/Canberra%20RR.htm mentions a Marconi NT201 transmitter and HR22 receivers, although the pictures show Racal RA17 receivers, not HR22s. Apparently in its original form, the Canberra had both a W/T room for the regular marine equipment and an R/T room for the ancillary equipment.
And per the attached item, the GPO cable-laying vessel HMTS Monarch was fitted with a Marconi NT201 ISB transmitter and a pair of NT203 wideband amplifiers. But then I guess that it [the Monarch] would have had business communications via the GPO point-to-point stations.
As far as I know the Marconi NT-series was primarily intended for naval applications; I imagine that some navies may have gone over to SSB/ISB before the merchant marine made its change to SSB.
Danielson & Mayoh, in the book “Marine Radio Manual” published in 1966, devoted a short chapter to SSB. They did not say in what situations it was being used, but simply made the general statement that the use of SSB equipment for marine communications was increasing. The worked example on the transmitting side wad said to be based upon a Marconi marine ISB unit. Possibly it was the NT201. The receiver worked example was not ascribed to any specific model, but it was of the ISB type with 2.9 MHz 1st IF, motor-driven AFC, and audio outputs for both sidebands, so it was not the Marconi HR22 for example. Both the transmitter and receiver were shown as having voice bandwidth (0.3 to 3 kHz) filters.
The QE2 was fitted with GPO Lincompex equipment, for which it had GEC RC410R receivers. Presumably Lincompex was used on telephone connections via GPO point-to-point stations, and not when communicating with marine shore stations. Working back from this, one might deduce that previous SSB/ISB installations on passenger vessels were linking with point-to-point stations, and not to marine shore stations.
Cheers,