Welcome to cruise-chat. Look here
for our guidance on dress codes, and also for a link to the official Carnival policy. Shorts were recently added as being acceptable on casual nights, although they do specify long dress shorts. Frankly, blue denim shorts that are neat will be allowed even if not mentioned in the dress suggestions.
for a link to the cash advance policy your asked about. You'd be better off bringing cash from home. Ships also have an ATM but it not reliable and also has high fees. You will need it anyway for your port visits. Tips are automatically added to your shipboard account at a rate of $10 per person/per day. This is the default action and these tips cover your cabin and dining room staffs. Room service is tipped in cash and I'd suggest you bring along a supply of dollar bills. The usual tip for a room service order is $2-$3, and perhaps as much as $5 for a large order.