Liked your point about
Castaway Cay. Have been there in January when it's been absolutely BEAUTIFUL, have been there when it's been OK and been there when we didn't get off of the ship because it was very windy and rainy and only 70 degrees (and therefore with the wind and rain - pretty cold).
We did a repo from Galveston to San Juan several years ago in January. On the sea days, it was chilly out on deck. Cozumel was a lovely day - we spent it onboard. We then went to Grand Cayman - and the waves were too dangerous for us to tender into the port. The captain got us close enough where we could see the waves going over the tender dock!! As we were sailing very slowly past, you could tell that it would have been a nice day on the island.
Galveston is notorious for having fog - which can cause issues for embarking and disembarking!! The cruise the week before ours was delayed by 8 hours arriving and was not allowed to depart until the following day. When it returned (our cruise) it was a few hours late and we were asked to delay our arrival at the port by two hours. Within the past two weeks the Wonder was "stuck at sea" because the port was closed (wind i think)
so yea, there can be weather issues where ever and whenever you go!!!
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Like others have said ... Longer is usually better.
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Flying to Houston then getting to Galveston can be spendy since the rental car places charge a huge drop off fee (still trying to recoup the cost of a ruined fleet of cars from the hurricane in 2008).
DCL has transfers from/to IAH, but not Hobby. There are many towncar and shuttle services between Houston and Galveston. What you choose to do depends upon when you want to arrive (day before cruise - PREFERRED! or day of AND where do you want to stay if the day prior - Houston or Galveston).
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I've been to Coz and Grand Cayman enough times where I don't really care if i get off of the ship or not - especially if I need to get my "Ship Fix".
If you want a beach day, Grand Cayman has many a great beach. Snorkeling - yes several options, all pretty good - I've done 3 different snorkeling excursions through DCL.
Cozumel also has good beaches and "resorts" on the beach with many activities. I've been over to Playa del Carmen a couple of times and really enjoy XPLOR park (Zip Lines, Drive Jeeps, Swimming in caverns, lunch ...) but there have been some travel advisories regarding taking the ferry between Coz and Playa ... so I don't know if DCL is still offering excursions that go to the mainland.
Decisions, Decisions.
Good Luck and Have Fun!