Most people would say it is obvious, but have YOU sailed across the Atlantic. I did not think so (well Mary has). So here comes the similarities first.

In both sailing and flying you tend to mention the passage time, not the time to get you to the departure. 

In both cases you prepare by packing, bad sleeping before departure and you are tired when you get there.

Both require safety check, but you do it yourself when sailing

Both have life vests on board

Both have bags for seasickness (one of them is smaller)

You need no boat

You need not to prepare for three years

Food and beverages are served to you by people and not by waves

No hassle with navigation and weather (unless the captain asks for help)

It is usually faster 

Same climate the whole journey (but bad)

The kids are strapped with safety belts

No falling in the water unless there is a MAJOR problem

Do not have to listen to the safety instructions

Plenty of room for your legs

No jetlags

You can decide when to leave (but not when to arrive)

You get a lot of fresh air (or sea spray)

You get to see a lot of water (more than you wish for)

You get a lot of patience 

No hassle with other passengers that you do not know (unless you have problems with the ones you know)

The kids can play as they like 

Can't see much more difference!