Remember that if OK Plus goes out of business, you will in all probability cease to be SkyTeam ElitePlus.

Note that it's hard to redeem DL miles well in terms of both (especially low) availability and the time/hassle to actually book the award ticket.

When comparing the ship fare, don't forget to include the price of getting to the port, taxes and port fees that might not be included in the fare, tips, and miscellaneous expenses on board such as internet access.