Carnival tipping is handled as follows: all bar sales automatically have 15% added to the drink price. So when you order a beer for example, the price on the menu may say $5.25 but when you get the receipt it will say $6.04. Those tips are for the bar staff. Tips for other staff (dining, cabin, buffet, others) are automatically added to your onboard account at the rate of $11.50 per person/per day. So, if there are two of you going your onboard account will show $115 for gratuities. You can always tip extra if you wish, but otherwise all tips are handled automatically.
Carnival classifies there dress codes as 'cruise casual' and 'cruise elegant'. On a 5-night cruise you will have one 'cruise elegant' night and the others will be 'cruise casual'. They are fairly relaxed about it. You can even wear shorts (not too short though) on the casual nights in the dining room. If you look here
you will find the dress suggestions.
Motion sickness is subjective. If you are prone to it you likely already know it (car sickness for example). Otherwise, motion on a ship is usually not a problem.