The included wine selection on Coral Expeditions is very ordinary, but it is possible to buy better wines from a reasonable selection at reasonable price. We tended to drink the included wines at lunch and pre-dinner, but buy something better with dinner. It was buy-as-you-go, rather than a credit.
Just surmising, I can't imagine too many cruise ships having high-level (ie. expensive) wines on their inclusive selection owing to non-drinkers feeling as though they are paying high voyage prices to subsidise drinkers. I'd guess that the operators are treading a careful line in trying to give an impression in their promotional material of 'sophistication' and having good included booze while being sensitive to non-imbibers muttering.
I constantly return to my motto: 'Always be prepared to be disappointed.'
When I went to Antarctica with OneOcean Expeditions and to the Arctic with Heritage Expeditions, both very much more spartan expedition vessels, it was all buy your own.
When I went on the Kimberley coast trip, it was on a 12-pax 26m unlicensed vessel operating out of Broome and we took our own.*
*Edit: If I had known in advance just how sensational the food was going to be, I would have taken some really good wines. In the crew of five was an exceptional chef. We foraged from the sea quite a lot (fresh-collected oysters, fish (plenty of barras), mud crabs. Only took what we needed; plenty of catch-and-release) but also had steaks, roast pork and so on. We had 10 pax, of a possible 12.
. We saw a splash of a dugong (supposedly ... 
)
Champagne all round after a torrid 90 mins of cruising.
