AWO: Yacht-borne aviation (specifically helicopters) has been around since the early days of 'helicoptering'. Initially these were naval and coast guard exploits (and I am thinking of Anton Fletner's Kolibri which landed on German U-boats in the early 40's .. and there are other examples too).
In the civilian market I am guessing that 'helicopters-used-as-tenders' would have gained momentum with the development of large private yachts (aka superyachts) which have the capacity to accommodate a helideck. These emerged in earnest in the mid-to-late 70's with yacht-borne helicopters becoming more popular during the 80's and really 'taking-off' (so to speak) in the 90's, but there are certainly earlier examples, particularly of course on private exploration and fishing vessels.
There is an article about flying from yachts on page 2 which you can read
here.
'Yacht flying' is an 'established' practice and there are several companies which specialise in providing and managing such services. One such company is
Luviair of which Nigel Watson (ex-RN and ex-Sultan of Oman's Navy) is CEO. Another firm is
HeliRiveriera and several executive operators (such as Starspeed) have a 'yachting division' catering for those with yacht-borne requirements. At Starspeed this is headed by
Gary Butcher. The firm also has 'ex-yachters' such as Richard Poppe among its staff. You can contact any of these companies if you are interested in learning more about flying in such a role.
Yacht-borne (or yacht-based) flying is probably ideal for someone wishing to retire as, for the most part, the work is undemanding but (and it is a big but) this does vary according to the set-up of each particular operation. These days helicopter pilots in this role can be given a variety of functions to perform .. some, for example, are also the yacht's captain (as was the case with Nigel Watson above) while others are also the licensed aircraft mechanic and still others take-up different duties on-board the yacht.
A number of helicopters with regular 'yachting' duties are mainly used by the owner for onshore requirements so that the 'sojourns at sea' are merely temporary assignments within the overall flying requirement.
As with anything, find out as much as you can beforehand. Yacht-based flying (as glamorous as it may appear) is not always the easy ride it may seem and it certainly isn't everyone's cup of tea.