Welcome back, Vicki! You're correct that as a first time guest with Disney Cruise Line, you will be able to begin your online check-in at 75 days prior to sailing. Those guests who have sailed with Disney before (Castaway Club members) can begin checking in and booking adventures 90 or more days in advance, so at times, some adventures and onboard activities do fill up.
However, for the two Port Adventures that you mentioned, I can't guarantee it, but I don't forsee those being too full when you begin your check-in. Plus, since they both usually offer more than one departure time, you'll have more chances to get spots on the adventures. The
Seaworld Explorer Semi-Sub is not the same as the catamaran that guests will ride in the
Nassau By Land and Sea Adventure. The Semi-Sub is actually a submarine-like boat that guests climb down into. While it doesn't fully submerge underwater like a real submarine, and there are seats up top too, the entire bottom part of the semi-sub is underwater, where guests sit, and each guest has a porthole to view the beautiful underwater world! You'll feel like you're 10,000 Leagues Under The Sea! (Sorry, I just had to say it!) Have you ever been on the
Finding Nemo Submarine Voyage at Disneyland in California? The ride on the Semi-Sub reminds me of the Nemo Sub ride experience, although the boats are different. Whereas the catamaran in the Land and Sea adventure has above water seating for guests, so no undersea views on that boat, but it certainly is a great tour!
I hope those details help as you continue to plan your adventures, Vicki! Stop back if we can help with anything else!
Marcy