This isn't the only confusing thing about this scene - people also get confused about Varys suddenly turning up at the end. Half the answer to this I think is the same as to that - a heck of a lot of time has passed between this scene and the battle at Mereen (it'd be nice if they had communicated this more clearly than just having some sails in the fleet which, if you look closely, have Dornish and Tyrell sigils on them... but anyway...).
Enough time passed to:
- Repaint the sails of a whole fleet, and communicate with Varys somehow to agree a meeting spot for the three fleets (which would also be enough time to train the Dothraki in basic sailing skills, and become more familiar with these big wooden horses)
- Sail half way around the world and join up with the Dornish and Tyrell fleets (this would involve much more time at sea than Qarth to Astapor, so they'd all be much more experienced at this point than in any of the scenes from that journey)
There's also a big difference between being the first of your people to ever do something, and the second. As Dany put it last time, when they were retching their guts out and were all depressed and stuff:
Don't mock them. They're the first Dothraki to ever be on a ship, and they followed me across the poisoned water. If they follow me, others will too, with a true khalasar
Last time, they were genuinely terrified, travelling over the notorious poisonous waters in a strange, rickety wooden horse, doing something completely alien to their people. This time, those survivors from the Khal Drogo days have been there, done that, and can set an example for the others.
Dothraki follow strength above all else
It also helps that this time, they're part of a strong khalasar going to war, as opposed to last time, when they were aimless survivors: feeling down before they even got on the ship. As Jorah said in the same scene:
You'll have a true khalasar when you've proved yourself strong, and not before
And this time, they are a true khalasar with a leader who proved herself strong, riding to war like thousands of Dothraki have done proudly for centuries before them. They may be doing it in an unusual way, with wooden horses instead of horsemeat ones, but this new way is tried and tested now. By the time this scene happens, they've had plenty of time at sea to get used to it and (figuratively and literally) learn the ropes.
It'd be strange if this time, they weren't posing, acting tough, trying to out-sailor each other, competing to be the best wooden-horse rider in the army, mocking the weak who still struggle.
But yeah, it'd have helped if they'd made the massive time jump before this scene more apparent...