What do you consider "luxury?"
Space? In general, more individual space, more $$$
Privacy? Ability to be alone? Again, more is more $$$
King sized beds & a private head? $$$
Good interior climate control? Generally, the more control you have, the more $$$
Speed & ride? Faster & smoother is more $$$
Quiet? The quieter your quarters, the more they cost.
Warm water down your wetsuit at the end of a dive? $$$
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For me, "luxury" is having as many dives as I can stand, a place to sleep, an available head, plenty to eat, good company and plenty of rum.
Blackbeards;
Spree;
Fling all the way
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If getting in a bunch of dives isn't as important as luxury on a budget, you can take a very economical cruise like
this one and have your own stateroom, private head with shower, lots of entertainment, and eat your way across the sea, then tack on a dive when in port...
Rick