Chris - thanks for pointing that out! I was typing as I was falling asleep last night, so my reply was slightly badly phrased... But even with a transportable licence for the handheld radio, it should still only be used on a vessel/tender, and not by the wife at home in the galley!
I agree on PMR not being highly useful, for various reasons. We used them when I had my first RIB as a teenager, and even within Poole Harbour range was dreadful, many sets don't handle getting wet, and because most are designed for use on land we found them hard to hear over the outboard. But that was 10 years ago, with relatively early PMR's, so things might have changed a bit now.
I'd always have some form of VHF onboard both in case I have an emergency, and so I can hear broadcasts from others, but for the original question, a cheap throw away mobile phone sounds like the best, and probably the cheapest, option for contacting the wife!